An Etymological Dictionary of Astronomy and Astrophysics

The exterior part of the → solar corona, illuminated by solar light scattered or reflected by dust particles. The same phenomenon also produces the → zodiacal light,
much farther away from the Sun. The dust particles are at most several microns in size and make up a disk stretching over almost 1 solar radius (700,000 km) from the Sun’s surface. Unlike electrons, which are responsible for the → K corona, the dust particles move relatively slowly. Thus, the light scattered by them has the same spectrum as the → photosphere and shows the → Fraunhofer lines. The F corona is the most luminous part of the corona at 1.5 solar radii from the Sun’s surface (M.S.: SDE).

See also: F referring to the Fraunhofer lines; → corona.

Waves that propagate only on the stellar surface. See also:
→ oscillation mode; → g mode; → p mode.

See also: f referring to → fundamental; → mode.

The → Saturn’s ring, with a width of 30-500 km, lying just outside of the → A ring, at 140,210 km from the center of Saturn.

See also: → ring.

Same as → focal ratio.

See also: f, from → focal; → number.

A star whose spectrum is characterized by strong → absorption lines of ionized → calcium, Ca II (→ H and K lines), which become much stronger than the hydrogen lines of the → Balmer series. A multitude of fainter metallic lines are also present. Ca II lines strengthen to later types.
→ Main sequence F stars, of which → Procyon is an example, have a → surface temperature of 6,000 to 7,400°C and a mass of 1.1 to 1.4 → solar masses (Habets & Heintze, 1981, AASS 46, 193).

See also: B, letter of alphabet used in the → → Harvard classification; → type; → star.

An extension of Einstein’s → general relativity derived from relaxing the hypothesis that the → Hilbert-Einstein action for the → gravitational field is strictly linear. This was done by
replacing the → Ricci scalar,
R, with a non-linear function of R:
S = (1/2κ)∫d⁴x (-g)^1/2f(R) + S^m,
where κ ≡ 8πG and S^m is the matter part of the action. The case of f(R) = R represents the simplest type of f(R) gravity theories. The discovery of → dark energy in 1998 stimulated the idea that → cosmic acceleration today may originate from some modification of gravity to general relativity. Dark energy models based on f(R) theories have been extensively studied as the simplest modified gravity scenario to realize the late-time acceleration. There are three versions of f(R) modified gravity: metric (or second order) formalism, Palatini (or first order) formalism, and metric-affine gravity.

See also: f(R), function of the → Ricci scalar; → gravity.

An empirical power-law correlation between the luminosity (L) and the velocity dispersion of stars (σ) in the center of a elliptical galaxies.
The original relation can be expressed mathematically as: L ∝ σ^γ, where the index γ is observed to be approximately equal to 4, but depends on the range of galaxy luminosities that is fitted. → Tully-Fisher relation.

See also: After the astronomers Sandra M. Faber and Robert Earl Jackson, who
first noted this relation in 1976 (ApJ 204, 668); → relation.

1. A short tale to teach a moral lesson, often with animals or inanimate objects as characters.
   

2. A story not founded on fact.
   

3. A story about supernatural or extraordinary persons or incidents; legend (Dictionary.com).

Etymology (EN): M.E. able, fabel, fabul, from O.Fr. fable “story, fable, tale; drama, play, fiction; lie, falsehood,” from L. fabula “story, story with a lesson, tale, narrative, account; the common talk, news,” literally “that which is told,” from fari “speak, tell,” from PIE root *bha- “speak.”

Etymology (PE): Afsâné, from Proto-Ir. *abi-sanhana-, from *sanh- “to declare, explain;” cf. O.Pers. θaⁿh- “to declare, say;”
Av. səngh- “to declare;” → speech.

A type of interferometer wherein the beam of light undergoes multiple reflections between two closely spaced partially silvered surfaces. Part of the light is transmitted each time the light reaches the second surface, resulting in multiple offset beams which can interfere with each other. The large number of interfering rays produces an interferometer with extremely high resolution, somewhat like the multiple slits of a diffraction grating increase its resolution.

See also: The design was conceived by French physicists Charles Fabry (1867-1945) and Alfred Pérot (1863-1925) in the late nineteenth century; → interferometer.

1. To tell invented stories; create fables or stories filled with fantasy.
   

2. To relate an event as a fable.

Etymology (EN): From L. fabulatus perfect passive participle of fabulor, from fabula, → fable.

Etymology (PE): Afsâne-bâftan “to forge fables, stories,”
from afsâné, → fable, + bâftan “to weave, twist, plait,” → texture.

1. To tell invented stories; create fables or stories filled with fantasy.
   

2. To relate an event as a fable.

See also: → fabulate; → -tion.

1. Almost impossible to believe; incredible.
   

2. Exceptionally good or unusual; marvelous; superb.

See also: → fable; → -ous.

1. The front part of the head, from the forehead to the chin.
   

2. A plane surface of a geometric solid. The front of something having more than two sides. → interface.

Etymology (EN): M.E., from O.Fr. face, from L. facies “appearance, form; visage, countenance.”

Etymology (PE): Dim “face,” from Av. daēman- “eye,” from dā(y)- “to see,” didāti “sees” (cf. Mod.Pers. didan “to see,” Mid.Pers. ditan “to see, regard, catch sight of, contemplate, experience;” O.Pers. dī- “to see;” Skt. dhī- “to perceive, think, ponder; thought, reflection, meditation,” dādhye; Gk. dedorka “have seen”).
Ru(y), variant rox
“face, surface; aspect; appearance” (Mid.Pers. rôy, rôdh “face;” Av. raoδa- “growth,” in plural form “appearance,” from raod- “to grow, sprout, shoot;” cf. Skt. róha- “rising, height”).

An angle formed by any two adjacent edges of a → polyhedron, in contrast to a → dihedral angle.

See also: → face; → angle.

A → spiral galaxy oriented such that it is viewed from above or below. → edge-on galaxy.

See also: → face; → on-; → galaxy.

To make easier, render less difficult.

Etymology (EN): From Fr. faciliter “to make easy,” from stem of L. facilis “easy to do,” from facere “to do,” → fact.

Etymology (PE): Âsânidan “to render easy,” from âsân, → easy.

The act or process of facilitating.

See also: → facilitate; → -tion.

1. Ease in moving, acting, or doing.
   

   2. Something that facilitates an action or process.
      

   3. A building, room, array of equipment, or a number of such things, designed to serve a particular function (TheFreeDictionary.com).

Etymology (EN): → facile; → -ity.

Etymology (PE): Âsânâk, from âsân “easy,” + relation suffix -âk, as in xorâk, pušâk, dârâk.

Something that has actual existence; a piece of information presented as having objective reality. → scientific fact.

Etymology (EN): L. factum “event, occurrence,” literally “something done, deed,” from neut. p.p. of facere “to do” (cf. Fr. faire, Sp. hacer), from PIE base *dhe- “to put, to do” (cf. Mod.Pers. dâdan “to give;” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives; puts;” Skt. dadáti “puts, places;” Hitt. dai- “to place;” Gk. tithenai “to put, set, place;” Lith. deti “to put;” Czech diti, Pol. dziac’, Rus. det’ “to hide,” delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do”).

Etymology (PE): Bâšâ, from bâš + -â agent suffix; bâš, present stem of budan “to be,” from
Mid.Pers. budan, from O.Pers./Av. bav- “to be; become, take place;” Av. buta- perf. ptcpl. pass., bavaiti “becomes”
(cf. Skt. bhavati “becomes, happens,” bhavah “becoming; condition, state;” PIE *bheu- “to be, come into being, become;”
Gk. phu- “become,” phuein “to bring forth, make grow;” L. fui “I was” (perf. tense of esse), futurus “that is to be, future;” Ger. present first and second person sing. bin, bist; E. to be; Lith. bu’ti “to be;” Rus. byt’ “to be”); budé also from budan.

1. One that actively contributes to the production of a result.
   

2. Math.: Any of the numbers or symbols that when multiplied together form a → product.

Etymology (EN): M.Fr. facteur “agent, representative,” from L. factor
“doer or maker,” from facere “to do” (cf. Fr. faire, Sp. hacer); from PIE base *dhe- “to put, to do;” cf. Skt. dadhati “puts, places;” Av. dadaiti “he puts;”
Hitt. dai- “to place;” Gk. tithenai “to put, set, place;” Lith. deti “to put;” Rus. det’ “to hide,” delat’ “to do;” O.H.G. tuon; Ger. tun; O.S., O.E. don “to do.”

Etymology (PE): Karvand, from kar- root of Mod.Pers. verb kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”)

• -vand a suffix forming adjectives and agent nouns.

A diagram representing a systematic way of determining all the prime factors of a number.

See also: → factor; → tree.

1. (n.) The product of all the positive integers from 1 to n, denoted by symbol n!
   
2. (adj.) of or pertaining to factors or factorials.

See also: → factor + -ial, from L. -alis,
→ -al.

The operation of resolving a quantity into factors.

See also: → factor + → -ize.

A bright area of the → photosphere of the Sun visible in white light and best seen near the solar limb, although they occur all across the Sun. Faculae raise several hundred kilometers above the photosphere and are associated with → sunspots. They often appear immediately before the formation of a sunspot group and remain visible for several days or weeks after the disappearance of the spots. Faculae are formed when a strong magnetic field heats a region of the photosphere to higher temperatures than the surrounding area.

Etymology (EN): Facula, from L. fac-, fax “torch” + -ula, → -ule.

Etymology (PE): Perisk, periska, biriske in Lori, Laki, and Kurd. dialects “spark” (Lârestâni pelita), probably related to Lori porpor “blazing charcoal,” Gilaki bur, biur “smokeless red fire;” cf. Tokharian por, puwar “fire;” Gk. pyr “fire;” Hitt. pahhur “fire;” Skt. pū- “to cleanse;” E. fire; O..E. fyr, from P.Gmc. *fuir (cf. O.N. fürr, M.Du. vuur, Ger. Feuer); PIE base *paewr- “fire.”

A temperature scale (°F) in which the → freezing point of → water is 32 degrees and the → boiling point is 212 degrees; the points are placed
180 degrees apart. It converts to the → Celsius scale by the formula: C = (5/9)(F - 32). See also → Kelvin scale, → Rankine scale, → Reaumur scale.

See also: Developed by the German physicist Gabriel Fahrenheit (1686-1736); → scale.

1. To be unsuccessful in achieving something expected, attempted, or desired.
   

   2. To neglect to do something.
      

   3. To be e unsuccessful in an examination.

Etymology (EN): M.E. failen, from O.Fr. falir “be lacking, miss, not succeed; come to an end; make a mistake; be dying,” from Vulgar L. *fallire, from L. fallere “to trip, cause to fall;” figuratively “to deceive, trick, dupe, cheat; fail, be lacking or defective.”

Etymology (PE): Qâvidan, from Choresmian γaw “to fail, to commit a fault;” cf. Av. gau- “to commit a sin;” Parthian (+*fra-) pargaw- “to owe, to lack;” Ossetian qaewyn/qyd “to be in need of something, to lack;” Proto-Ir. *gaHu- “to be faulty, wanting; to need;” PIE *g^heH<SUB2u- “to be faulty, lacking, insufficient;” cf. Old L. hauelod “insufficient, false,” L. hau(d) “not;” O.Irish gáu, gó; Welsh gau “lie” (Cheung 2007).

1. An act or instance of failing; lack of success.
   

   2. A state of inability to perform a normal function.

See also: Verbal noun from → fail.

1. Lacking brightness, vividness, clearness, loudness, strength; feeble; exhausted.
   

   2. To lose consciousness temporarily; to lose brightness (Dictionary.com).

Etymology (EN): From O.Fr. faint, feint “false, deceitful; sham, artificial; weak, faint, lazy,” p.p. of feindre “hesitate, falter, show weakness,” from L. fingere “to touch, handle; change.”

Etymology (PE): Tâm, from Pers. tâm “feeble, fragile, weak,” ultimately from Proto-Ir. *tamH- “to faint, be tired;” cf. Khotanese ttām&#257 “fatigue;” Parthian t’m’dg “fainted, choking;” Gilak (Langarudi) tâmâ, tâm “silent;” PIE base *temH- “to faint, to be dark;” tâmidan, infinitive from tâm.
Nazâr, from Mid.Pers. nizâr “weak, feeble” (variant zâr), zarmân “old man, deterioration;” Av. zairina- “exhausting, slackening,” zaurura- “weak through old age, decrepit;” cf. Skt. jára- “wearing out, exhaustion,” jaranā- “old, decayed,” jarimán- “weakness through old age,” Gk. geron “old man,” L. granum “grain;” PIE base *ger- “wear away.”
Kamtâb, from kam “little, few; deficient, wanting; scarce,” from Mid.Pers. kam “little, small, few,” O.Pers./Av. kamna- “small, few”

• tâb “light; heat, warmth; illuminating,” from tâbidan, tâftan “to shine,” tafsidan “to become hot;” Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth;” cf.
  Skt. tap- “to spoil, injure, damage; to suffer; to heat, be/become hot,” tapati “burns;” L. tepere “to be warm,” tepidus “warm;” PIE base *tep- “warm.”
  Kamnur, from kam, as above, + nur, → light.

The contradiction between a colder Sun (about 30% less luminous) some 4 billion years ago, as predicted by models, and the warm ancient Terrestrial and Martian climates derived from geological evidence.

See also: → faint; → early; → sun; → paradox.

For unaided eye, a star of visual magnitude around 5-6. Otherwise, on an image, a star that has a lesser brightness compared to others of the same field.

See also: → faint; → star.

1. Confidence or trust in a person or thing.
   

   2. Belief that is not based on proof.
      

   3. A system of religious belief (Dictionary.com).

Etymology (EN): M.E. feith fei, fai “faithfulness to a trust or promise; loyalty to a person; honesty, truthfulness,” from Anglo-Fr. and O.Fr. feid, foi “faith, belief, trust, confidence; pledge,” from L. fides “trust, faith, confidence, reliance, credence, belief,” from root of fidere “to trust,“from PIE root *bheidh- “to trust, confide, persuade.”

Etymology (PE): Imân, loan from Ar. al-imân “faith, belief, trust.”

1. Something that is not what it seems to be; counterfeit.
   

   2. To forge or counterfeit; pretend.

Etymology (EN): Of unknown origin.

Etymology (PE): Mâx (Dehxodâ) “counterfeit, not genuine, forged, adulterated (gold or silver);” Mid.Pes. mih “false; contrary;” Av. maēθā- “deviating; changeable,” miθō “falsely, wrongly;” maybe related to mâz “a wrinkle, twist, fold;” (Khonsâri, Natanz-Toroqi) max “unstable, volatile.”

A collected meteorite whose arrival on Earth is witnessed, as opposed to a → find.

Etymology (EN): M.E. fallen, from O.E. feallan, from P.Gmc. *fallanan (cf. O.N. falla, O.H.G. fallan), from PIE base *phol- “to fall” (cf. Arm. p’ul “downfall;” Lith. puola “to fall”).

Etymology (PE): Oft, stem of oftâdan “to fall;” Mid.Pers. opastan “to fall,” patet “falls;” Av. pat- " to fly, fall, rush," patarəta- “winged;” cf. Skt. patati “he flies, falls,” pátra- “wing, feather, leaf;” Gk. piptein “to fall,” pterux “wing;” L. penna “feather, wing;” O.E. feðer “feather;” PIE base *pet- “to fly, rush.”

Logic: An → error in → reasoning that renders an → argument logically → invalid such as affirming the → consequent and → denying the → antecedent.

Etymology (EN): From L. fallacia “deception,” from fallere
“to deceive, trick, cheat; fail, be defective.”

Etymology (PE): Titâl (Dehxodâ) “deceit; deceiving speech, fallacious words;” cf. Tabari titâl hâ kərdan “to deceive (somebody) wheedlingly,” Pashtu titâl “duplicity, guile, deceit, fraud.”

1. Not true or correct; erroneous.
   
2. Not properly, accurately, or honestly made, done, or adjusted.
   → false color, → false dawn, → false nucleus, → false vacuum.

Etymology (EN): From O.Fr. fals, faus, from L. falsus “deceived, erroneous, mistaken,” p.p. of fallere “to deceive, disappoint.”

Etymology (PE): Zif, from Tâleši saf “wrong,” from Mid.Pers. zêfân, zaspân “wrong, vile;” in classical Pers. dictionaries zif “churlishness; sin.”

In imaging technique, assigning color to black and white images to differentiate features or convey information. → true color.

See also: → false; → color.

An unusually early glow of the horizon near the rising sun during certain times of the year. This early glow does not originate directly from the Sun, but is rather caused by → zodiacal light. It may be mistaken for a sunrise.

See also: → false; → dawn.

An especially concentrated region in the → coma of some → comets, representing the dense cloud of inner coma rather than the much smaller true nucleus. Also called apparent nucleus and → pseudo-nucleus.

See also: → false; → nucleus.

A peculiar, hypothetical state of matter which is predicted to exist by current theories of → elementary particles, including the → grand unified theories. Unlike the ordinary vacuum, a false vacuum has a large → energy density and a large → negative pressure. A false vacuum is the driving force behind the rapid expansion in the → inflationary model of the → Universe.

See also: → false; → vacuum.

Philosophy of science: The concept according to which a proposition or theory cannot be scientific if it does not admit consideration of the possibility of its being false. According to Karl Popper (1902-1994), falsifiability is the crucial feature of scientific hypotheses. Any theory not falsifiable is said to be unscientific.

See also: → falsifiable; → -ity.

The quality of something that can be falsified. → falsifiability.

See also: → falsify; → -able.

1. To show or prove that a theory is incorrect or false.
   

2. To alter something in order to deceive.

See also: Verb from → false.

The quality or condition of being → false. Opposite of → truth.

See also: → false; → -ity.

A group of entities with similar properties and common origin.

→ family of curves, → family of distributions, → family of sets, → comet family.

Etymology (EN): From L. familia “household, the slaves of a household,” from famulus “servant,” of unknown origin.

Etymology (PE): Xânevâdé “family,” from xâné “house, home, houshold”

• vâdé “root, foundation, origin.” Xâné, from Mid.Pers. xânak, xân, xôn; cf. L. cunae “cradle;” Gk. kome “village;” Pers. Aftari dialect kiye “house, home;” PIE base *kei- “bed; to lie, to settle; beloved” (other cognates: P.Gmc. *khaim-; O.E. ham “dwelling, house, village;” E. home; Ger. Heim; L. civis “townsman;” Fr. cité; E. city; Skt. śiva- “auspicious, dear”).

A set of similar curves which are distinguished by the values taken by one or more parameters in their general equation. For example, the general solution of a differential equation is represented by a family of curves.

See also: → family; → curve.

A set of distributions which have the same general mathematical → formula.

See also: → family; → distribution.

A → collection of → subsets of a set.

See also: → family; → set.

In 3D → magnetic reconnection models of solar plasma, a plane or curve surface composed of magnetic field lines emanating from the → magnetic null point (almost radially in the absence of electric currents and spirally if electric currents are present). See also → spine (Lau & Finn. 1990, ApJ 350, 672; Parnell et al. 1996, Physics of Plasmas 3, 759).

Etymology (EN): M.E., from O.E. fann, from L. vannus “a basket or shovel for winnowing grain,” related to ventus, → wind.

Etymology (PE): Bâdzan “fan, ventilator,” from bâd, → wind + zan from zadan “to strike, beat; to play an instrument; to do” (Mid.Pers. zatan, žatan; O.Pers./Av. jan-, gan- “to strike, hit, smite, kill” (jantar- “smiter”); cf.
Skt. han- “to strike, beat” (hantar- “smiter, killer”);
Gk. theinein “to strike;” L. fendere “to strike, push;” Gmc. *gundjo “war, battle;” PIE *g^when- “to strike, kill”).

In the → Fanaroff-Riley classification, sources with R_FR < 0.5. Fanaroff and Riley (1974) found that nearly all sources with luminosity L(178MHz) ≤ 2 × 10²⁵h₁₀₀^-2 W Hz^-1 sr^-1 were of class I. FR-I → radio jets are thought to be → subsonic, possibly due to mass entrainment, which makes them amenable to distortions in the interaction with the ambient medium.

See also: → Fanaroff-Riley classification; → class.

In the → Fanaroff-Riley classification, → radio sources with hotspots in their lobes at distances from the center which are such that R_FR > 0.5. The → radio jets in FR-II sources are expected to be highly → supersonic, allowing them to travel large distances.

See also: → Fanaroff-Riley classification; → class.

A classification scheme for distinguishing a → radio galaxy from an → active galaxy based on their → radio frequency  and → luminosity and their kpc-scale appearance. Analyzing a sample of 57 radio galaxies from the → 3CR catalogue, which were clearly resolved at 1.4 GHz or 5 GHz, Fanaroff & Riley (1974) discovered that the relative positions of regions of high and low → surface brightness in the → lobes of extragalactic → radio sources are correlated with their radio luminosity. They divided the sample into two classes using the ratio R_FR of the distance between the regions of highest surface brightness on opposite sides of the central galaxy or quasar, to the total extent of the source up to the lowest brightness contour in the map. → Fanaroff-Riley Class I (FR-I) , → Fanaroff-Riley Class II (FR-II). The boundary between the two classes is not very sharp, and there is some overlap in the luminosities of sources classified as FR-I or FR-II on the basis of their structures. The physical cause of the FR-I/II dichotomy probably lies in the type of flow in the → radio jets.

See also: Bernard L. Fanaroff and Julia M. Riley, 1974, MNRAS 167, 31P; → classification.

Being at a great distance; remote in time or place.

Etymology (EN): O.E. feorr “to a great distance, long ago,” from P.Gmc. *ferro (cf. Du. ver, Ger. fern), from PIE *per- “through, across, beyond” (cf. O.Pers. para “on the other side (of);” Av. parə “beyond, more than, superior,” parô “except,” pərətu- “crossing, bridge;” Mod.Pers. pol “bridge;”
Skt. parás “far, further, beyond,” Gk. pera “across, beyond,” L. per “through”).

Etymology (PE): Dur, from Mid.Pers. dūr “far, distant, remote;” O.Pers. dūra- “far (in time or space),” dūraiy “afar, far away, far and wide;” Av. dūra-, dūirē “far,” from dav- “to move away;” cf. Skt. dūrá- “far; distance (in space and time);” PIE base *deu- “to move forward, pass;” cf. Gk. den “for a long time,” deros “lasting long.”

Infrared radiation in the wavelength range (25-40) to (200-350) microns emitted by cold molecular/dust clouds.

See also: → far; → infrared.

Ultraviolet radiation in the wavelength range 912-2000 Å. See also → extreme ultraviolet.

See also: → far; → ultraviolet.

The portion of the → electromagnetic radiation in the wavelength range between about 30 and 300 → microns. See also: → infrared radiation, → near-infrared, → mid-infrared, → submillimeter radiation.

See also: → far; → infrared.

The → SI unit of → capacitance,
defined as the capacitance of a conductor whose → potential
increases by one → volt when a charge of 1 → coulomb is imparted to it; symbol F.

See also: Named after the British physicist Michael Faraday (1791-1867), who made several major contributions to the fields of electricity and magnetism.

An enclosure made of conducting material, such as wire mesh or metal plates, that shields what it contains from external electric fields. According to → Gauss’s theorem, the electric field inside a hollow conductor is nil. In order to demonstrate this, Faraday, in 1836, made a large box covered with wire mesh, and went inside it himself with an → electroscope. Powerful charges were applied to the outside of the box, but he detected no effect inside the cage.

See also: → farad; → cage.

Same as → Faraday rotation.

See also: → farad; → effect.

The rotation of the plane of → polarization experienced by a beam of → linearly polarized radiation when the radiation passes through a material containing a magnetic field with a component in the direction of propagation. This effect occurs in → H II regions in which a magnetic field causes a change in the polarized waves passing through. Same as → Faraday effect.

See also: → farad; → rotation.

The induced → electromotive force in a circuit is equal in magnitude and opposite in sign to the rate of change of the → magnetic flux through the surface bounded by the circuit. Mathematically, it is expressed as: ∇ x E = -∂B/∂t, which is one of the four → Maxwell’s equations.

See also: → farad; → law; → induction.

An area of land devoted to the raising of animals, fish, plants, etc.

Etymology (EN): M.E. ferme “lease, rented land, rent,” from O.Fr., from Vulgar L. *ferma, derivative of *fermare for L. firmare “to make firm, confirm.”

Etymology (PE): Keštzâr “farm, field,” from kešt past stem of keštan, variants kâštan, kâridan “to cultivate, to plant;” Mid.Pers. kištan, kâridan “to sow, plant; to make furrows;” Av. kar- “to strew seed, cultivate,” kāraiieiti “cultivates;” cf. Skt. kar- “to scatter, strew, pour out,” + suffix -zâr denoting profusion, abundance, as in kârzâr “a field of battle; combat” šurezâr “unfertile, salty ground; nitrous earth,” xoškzâr “arid land,” and so forth.

Moving or able to move, operate, function, or take effect quickly; quick; swift; rapid (Dictionary.com).

Etymology (EN): M.E., from O.E. fæst “firmly fixed, steadfast;” O.Fr. fest, O.N. fastr, Du. vast, Ger. fest.

Etymology (PE): Tond “swift, rapid, brisk; fierce, severe,” → velocity.

A bright → burst of → radio emission lasting only a few milliseconds, and thought to be of → extragalactic origin. The first ever detected such burst, called the → Lorimer burst, was in 2007. It lasted only 5 milliseconds, but the single radio → pulse was dispersed over a wide range of frequencies (→ dispersion measure). This suggested a → cosmic origin for the burst, because the radiation must have passed through very distant → intergalactic clouds to be so highly dispersed. The second FRB was detected in 2012 in archival data from the Parkes Radio Telescope, the same telescope through which the original burst was seen. No temporally coincident → X-ray or → gamma ray signature was identified in association with the bursts. Most recent results suggest FRBs as a new population of explosive events at cosmological distances of up to 3 → giga  → parsecs, that is → redshifts of 0.5 to 1. While physical interpretations for this phenomenon remain speculative, they are thought to involve highly → compact objects, such as → neutron stars. See also → blitzar.

See also: The term fast radio burst was coined by Thornton et al., 2013, Science, 341, 53 (arXiv:1307.1628); → fast; → radio; → burst.

1. To attach firmly or securely in place; fix securely to something else.
   

2. To make secure, as an article of dress with buttons, clasps, etc., or a door with a lock, bolt, etc. (Dictionary.com).

Etymology (EN): From M.E. fastenen, from O.E. fæstnian; cognate with O.Fris. festnia “to make firm, bind fast,” O.Sax. fastnon, O.H.G. fastnion, O.N. fastna “to pledge, betroth.”

Etymology (PE): Darizidan, from Proto-Ir. *darz- “to attach, fasten;” cf. Av. darəz- “to attach;” Mid.Pers. handarz “advice, order, command,”
drz- “to fasten;” Mod.Pers. andarz “advice; testament,” darzan “needle,” darzi “tailor,” razé (with elimination of the initial phoneme) “a ring or staple used to fasten a door,” padarzé “a wrapper in which clothes are folded up;” cf. Skt. drah- “to fix, make firm;” Gk. drassomai “I take hold of, grasp;” Russ. deržat’ “to hold, keep” (Cheung 2007).

A male → parent.

Etymology (EN): M.E. fader, from O.E. fæder “father, male ancestor;” cf. O.S. fadar, Du. vader, O.N. faðir, O.H.G. fater, Ger. Vater; PIE *pəter-; cognate with Pers. pedar, as below.

Etymology (PE): Pedar, from Mid.Pers.
pidar, variant pid “father;”
O.Pers. pitā- “father;” Av. patar-; cf. Skt. pitár-; Gk. pater; L. pater (Fr. père, Sp. padre).

Geology: A fracture in the Earth’s crust along which the adjacent rock surfaces have been displaced relative to each other. Movement along the fault can cause → earthquakes or, in the process of mountain-building, can release underlying → magma and permit it to rise to the surface as a volcanic eruption.

Etymology (EN): M.E. faute “deficiency,” from O.Fr. faute “opening, gap; failure, flaw; lack,” from V.L. *fallita “a shortcoming, falling,” from L. falsus “deceptive, feigned, spurious,” p.p. of fallere “to deceive, be wrong.”

Etymology (PE): Gosalé, noun from gosalidan “to break; to snap asunder,” ultimately from Proto-Iranian *uisar-, from *ui- “apart” + *sar- “to break;” cf. Av. sairi- “fragment,” asarəta- “not broken;” Skt. sar- “to break, tear apart,” śūrtá- “smashed,” aśīrtá- “unharmed;” Gk. keraizo “to tear, destroy,” akeraios “unharmed;” PIE base *ker- “to hurt, harm.”

Geology: The surface of a fracture along which dislocation of adjacent rocks has taken place.

See also: → falt; → surface.

The geological process leading to the formation of → faults.

See also: Verbal noun, → fault.

One of light appendages that grow from a bird’s skin and form its covering.

Etymology (EN): M.E., from O.E. fether; akin to Du. veder, Ger. Feder, O.N. fioþr, Sw. fjäder, from PIE root *pet- “to rush, to fly.”

Etymology (PE): Parr “feather,” variant bâl “wing,” Mid.Pers. parr “feather, wing,” bâl; Av. parəna- “feather,” Skt. parnam, cf.
O.H.G. farn “fern,” PIE pornom “feather.”

A prominent or conspicuous part or characteristic. → absorption feature; → coronal features;
→ dust feature; → emission feature.

Etymology (EN): From O.Fr. faiture “fashion, shape, form,” from L. facura “a formation,” from facere “to make, do, perform” (cf. Fr. faire, Sp. hacer), from PIE base *dhe- “to put, to do” (cf. Mod.Pers. dâdan “to give;” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives; puts;” Skt. dadáti “puts, places;” Hitt. dai- “to place;” Gk. tithenai “to put, set, place;” Lith. deti “to put;” Czech diti, Pol. dziac’, Rus. det’ “to hide,” delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do”).

Etymology (PE): Ârang “color; mode, form, manner,” cf. Av. *iringa- “sign, mark” in haptôiringa- “with seven marks,” from hapto- “seven,” + iringa-; Mid.Pers. haptiring, Mod.Pers. haftowrang “the constellation of Great Bear;” cf. Skt. linga- “mark, token, sign.”

One of the most successful general methods for the numerical solution of the → radiative transfer equation.
This method has been primarily used to study → radiative transfer in the → photospheres of stars.

See also: P. Feautrier (1964), C.r. hebd. Séanc. Acad. Sci. Paris 258, 3198; → method.

See → Weber-Fechner law.

Biology: To impregnate, to fertilize.

Etymology (EN): L. fecundetus “made fruitful, fertilized,” p.p. of fcundare, from fecundus “fruitful, fertile, productive,” from L. root *fe-, corresponding to PIE *dhe(i)- “to suck, suckle;” cf. Skt. dhayati “sucks,” dhayah “nourishing;” Gk. thele “mother’s breast, nipple,” thelys “female, fruitful;” Mid.Pers. dâyag “(wet-)nurse;” Mod.Pers. dâyé “(wet-)nurse;” Proto-Iranian *daH- “to suck, suckle;” O.C.S. dojiti “to suckle,” dojilica “nurse;” Lith. dele “leech;” Goth. daddjan “to suckle;” O.H.G. tila “female breast.”

Etymology (PE): Gošnidan, from gošn “male,” Mid.Pers. gušn; cf. Av. varšni- “male;” Skt. vrsan-.

Biology: The act or process of fecundating.

See also: → fecundate; → -tion.

Relating to or characteristic of a unified body (e.g. a government) with constituent parts (states) that retain a measure of autonomy.

Etymology (EN): From Fr. fédéral, from L. foedus (genitive foederis) “league, treaty, alliance,” → federate, + → -al.

Etymology (PE): Hiyâvi, from hiyâvidan, → federate.

1. To join together in a → federation.
   
2. To organize on a → federal basis.

Etymology (EN): From L. foederatus “leagued together, allied,” p.p. of foederare “to establish by treaty,” from foedus
“league, treaty, alliance,” related to fides “faith.”

Etymology (PE): Hiyâvidan, from Tabari hiyâ “together, with each other,” probably related to Av. hi- “to chain, bind,” hiθav- “association, assemblage,”
hinav- “bond, chain,” hita- “fastened;”
O.P. vištāspa- literally “with unbound horses;” Av. hitāspa-; Pers. gošudan “to open,” → resolve; cf. Skt. sā-/say- “to bind, fasten, fetter.” Coined on the model of Ger. Bund “federation, alliance, band,” cognate with Pers. bastan “to bind,” → absolute.

A composition of different databases which work in an integrated manner while preserving their autonomy.

See also: → federate; → database; → system.

1. The act of federating or uniting in a league.
   

2. A political unit formed from smaller units on a → federal basis.

See also: Verbal noun of → federate.

Pertaining to or of the nature of a → federation.

See also: → federate.

He who, or that which, federates.

See also: Agent noun of → federate + → -or.

1. To give food to; supply with nourishment.
   

2. To furnish with necessary material.

Etymology (EN): M.E. feden, from O.E. fedan “to nourish;”
cf. O.S. fodjan, O.Fris. feda, Goth. fodjan “to feed.”

Etymology (PE): Xorândan, transitive form of xordan “to eat, consume,” Mid.Pers. xvardan “to eat, enjoy (food),” Av. x^var “to consume, eat;” Laki dialect hovârden “to eat;” Proto-Iranian *huar- “to consume, eat.”

1. For any system that has an → input and → output, the return of a fraction of the output to the input for the next action. Feedback process allows a system to regulate itself by monitoring its own output. It is of prime importance to the working of all regulatory mechanisms found in living and non-living nature, as well as in social systems such as education and economy.
   

2. By extension, any response or information about the result of a process.
   

3. → stellar feedback.
   

See also: → feedback loop, → negative feedback, → positive feedback, → radiative feedback, → feedforward.

See also: → feed; → back.

A closed transmission path in a → feedback process involving part of the → output as an → input for correction or control of the operation of a → system.

See also: → feedback; → loop.

In a self-regulatory system, monitoring a disturbance before it enters the → system to apply corrections before the disturbance has influenced the system. See also → feedback.

See also: → feed; → forward.

In a → radio telescope, a device located at the → focal point of the → antenna. It receives the → radio waves which the antenna collects and guides them to the → detector.

See also: → feed; → horn.

1. To perceive or examine by touch.
   

2. To have a sensation of (something), other than by sight, hearing, taste, or smell (Dictionary.com).

Etymology (EN): M.E. felen, from O.E. felan “to touch;” cf. O.S. gifolian, O.Fr.fela, Du. voelen, O.H.G. vuolen, Ger. fühlen “to feel;” from PIE root *pal- “to touch, feel, shake, strike softly” (cognates: Gk. psallein “to pluck (the harp),” L. palpare “to touch softly, stroke”).

Etymology (PE): Sohidan, from Mid.Pers. sôhistan “to feel, to touch,” sôhešn “feeling, sense,” of unknown origin. Note Pers. sahestan “to fear,” from Proto-Ir. *θrah- “to shake; to fear;” Pers. tars, harâs, sham “fear;” may be they are of different roots.

1. The function or the power of perceiving by touch.
   

2. Physical sensation not connected with sight, hearing, taste, or smell.
   

3. A particular sensation of this kind (Dictionary.com).

See also: Verbal noun of “to → feel.”

A group of rock-forming minerals whose members are the most abundant constituents of igneous rocks. They consist of aluminum silicates of potassium, sodium, and calcium and
typically occur as colorless or pale-colored crystals.

See also: Modification of obsolete Ger. Feldspath (now Feldspat), from Ger. Feld “field” + obsolete Ger. Spath (now Spat) “spar.”

1. Of, relating to, or being the sex that typically has the capacity to bear offsprings or produce eggs.

   2. A female person, animal, or plant.

Etymology (EN): M.E., alteration of femel, femelle, from O.Fr. femelle “woman, female,” from M.L. femella “a female,” from L. femella “young female, girl,” diminutive of femina “woman, a female,” → feminine.

Etymology (PE): Mâdé “female,” from Mid.Pers. mâdag, “female,” from mâd, → mother.

1. Characteristic of or appropriate or unique to women.
   

2. Of, relating to, or constituting the gender that ordinarily includes most words or grammatical forms referring to females (Merriam-Webster.com).

Etymology (EN): M.E. feminin, from O.Fr. femenin “feminine, female; with feminine qualities,” from L. femininus “feminine,” from femina “woman, female,” literally “she who suckles,” from root of felare “to suck, suckle;” cf. Gk. thele “mother’s breast, nipple,” thelys “female, fruitful;” Pers. dâyé, dâyah “(wet-nurse);” PIE root *d^he(i)- “to suck, suckle.”

Etymology (PE): Mâdin, from mâdé, → female.

1. Belief in the social, political, and economic freedom of women and equality of the sexes. Feminism is closely tied to democracy and → secularism.
   

2. The movement organized around this belief.

Etymology (EN): From Fr. féminisme, from féminin “feminine, female,” from L. femininus “feminine” (originally in the grammatical sense), from femina “woman, female,” literally “she who suckles,”
cognates fecund “fruitful, fertile,” felix “happy,” fetus “offspring, pregnancy;” PIE base *d^heh(i)- “to suck, suckle;” cf. Gk. thele “mother’s breast;” Pers. dâyé “wet nurse.”

Etymology (PE): Zâd-zan-bâvari, from zâdzan “free woman” (on the model of zâdmard “free man, valiant man, generous man,” zâdsarv “tall and upright cypress tree”), from zâd, contraction of âzâd,
→ free,

• zan, → woman, + bâvari, → -ism.

In the International System of Units, a prefix meaning 10^-15.

See also: From Danish and Norwegian femten “fifteen,” from O.N. fimmtān (Sw. femton, Du. vüftien, Ger. fünfzehn, E. fifteen, Pers. pânzdah), ultimately from
PIE base *penk^we “five” (cf. Mod.Pers. panj, Av. panca, Skt. pánca, Gk. pente, L. quinque) + PIE *dekm “ten” (cf. Mod.Pers. dah, Av. dasa, Skt. dáśa, Arm. tasn, Gk. deka, L. decem, Ger. zehn, E. ten, Fr. dix).

Any number of the form 2²ⁿ + 1, where n is a connective → integer. If Fermat number is → prime, it is called a → Fermat prime.

See also: → Fermat’s principle; → number.

A → Fermat number, 2²ⁿ + 1, that is a → prime number. The only known Fermat primes are: 3, 5, 17, 257, and 65537, corresponding to n = 0, 1, 2, 3, and 4.

See also: → Fermat’s principle; → prime.

In → number theory, the statement that for all → integers, the equation xⁿ + yⁿ = zⁿ has no solution in → positive integer.
After 358 years of effort by mathematicians to prove the theorem, a complete proof was found by Andrew Wiles in 1995.

See also: → Fermat’s principle; → last; → theorem.

The path taken by a ray of light going from one point to another through any set of media is such that the time taken is a minimum. This principle governs the light propagation and determines the geodesics of optical paths.

See also: Put forward by Pierre de Fermat (1601-1665), French mathematician, born at Beaumont-de-Lomagne; → principle

A unit of length equal to 10^-13 cm.

See also: After Enrico Fermi (1901-1954), the Italian-born American physicist who was a key figure in the development of nuclear physics. He was awarded the 1938 Nobel Prize for Physics.

The → coupling constant associated with the → weak interaction, which gives rise to → beta decay. C_F = 1.167 x 10^-5 GeV^-2.

See also: → Fermi; → constant.

The energy of the highest occupied quantum state in a system of fermions at absolute zero temperature. See also → Fermi sea.

See also: → fermi; → energy.

A space observatory, formerly named GLAST, devoted to the study of → gamma rays emitted from astrophysical objects.
Developed by NASA in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States, Fermi was launched on June 11, 2008. The main instrument, the Large Area Telescope (LAT), is an imaging → camera covering the energy range from about 20 → MeV to more than 300 → GeV. Such gamma rays are emitted only in the most extreme conditions, by particles moving very nearly at the → speed of light. The LAT’s → field of view covers about 20% of the sky at any time, and it scans continuously, covering the whole sky every three hours. Another instrument, the Gamma-ray Burst Monitor (GBM) has a field of view several times larger than the LAT and provides → spectral coverage of → gamma-ray burst that extends from the lower limit of the LAT down to 10 → keV.

See also: → Fermi; → gamma ray; → space; → telescope.

An old explanation, proposed by Enrico Fermi, of the → weak interaction.

See also: → fermi; → interaction.

A measure of the → energy of the least tightly held → electrons within a → solid at a → non-zero → temperature.
The value of the Fermi level at → absolute zero (-273.15 °C) is called the → Fermi energy and is a constant for each solid. In other words, the Fermi level is any → energy level having the probability that it is exactly half filled with electrons in the → Fermi-Dirac statistics. Levels of lower energy than the Fermi level tend to be entirely filled with electrons, whereas energy levels higher than the Fermi tend to be empty.

See also: → Fermi; → level.

The apparent contradiction between the high probability of the existence of extraterrestrial civilizations and the lack of evidence of contact with such civilizations.

See also: → fermi; → paradox.

Same as → degeneracy pressure.

See also: → fermi; → pressure.

A large aggregate of single-state → fermions of lowest energy. When the temperature is lowered to absolute zero, all electrons in solids attempt to get into the lowest available energy level. However, electrons cannot all occupy the lowest energy, or ground state, in virtue of the → Pauli exclusion principle. They stack up in the lowest energy states, with two fermions in each state, one spin up and one spin down. Such assemblage of filled states is called the Fermi “sea,” and this state of matter is called → degenerate. All states with energy less than the Fermi energy are filled, and all states above the Fermi energy are empty.

See also: → fermi; → sea.

The statistical distribution of → fermions over the energy states for a system in → thermodynamic equilibrium. In other words, the probability that a given energy level be occupied by a fermion.

See also: → fermi, → Dirac function; → statistics.

A computer experiment that was aimed to study the → thermalization process of a → solid. In other words, the goal was to see whether there is an approximate → equipartition of energy in the system, which would mean that the motion is → chaotic. Using computer simulation, Fermi-Pasta-Ulam studied the behavior of a chain of 64 mass particles connected by → nonlinear springs.

In fact, they were looking for a theoretical physics problem suitable for an investigation with one of the very first computers, the he MANIAC (Mathematical Analyzer, Numerical Integrator and Computer). They decided to study how a → crystal evolves toward → thermal equilibrium by simulating a chain of particles, linked by a quadratic interaction potential, but also by a weak nonlinear interaction.

Fermi-Pasta-Ulam assumed that if the interaction in the chain were nonlinear, then an exchange of energy among the normal modes would occur, and this would bring forth the equipartition of energy, i.e. the thermalization.

Contrary to expectations, the energy revealed no tendency toward equipartition. The system had a simple quasi-periodic behavior, and no → chaoticity was observed. This result, known as the Fermi-Pasta-Ulam paradox, shows that → nonlinearity is not enough to guarantee the equipartition of energy (see, e.g., Dauxois et al., 2005, Eur. J. Phys., 26, S3).

See also: E. Fermi, J. Pasta, S. Ulam, 1955, Los Alamos report LA-1940; → problem.

An elementary particle, such as → electron, → proton, or → neutron, having a half integral value of → spin. Fermions obey the → Pauli exclusion principle.

See also: From Fermi → fermi + → -on.

Of or containing → iron, especially in the trivalent state. More specifically, iron with an → oxidation number of +3; also denoted iron(III) or Fe³⁺.

See also: Ferric, from L. ferrum “iron,” + → -ic.

Iron in a plus-3 → oxidation state.
Ferric iron needs to share three electrons with an oxygen molecule to make the ion neutral.

See also: → ferric; → iron.

Indicating a property of → iron or the presence of iron.

Etymology (EN): Ferro-, variant ferri-, combining form of L. ferrum “iron.”

Etymology (PE): Âhan-, → iron.

Characterized by the presence of a spontaneous → electric dipole while not exposed to an external electric field. → ferroelectricity.

See also: → ferro-; → electric.

A property observed in certain materials characterized by the presence of a spontaneous electric polarization even in the absence of an external electric field. In the ferroelectric state the center of positive charge of the material does not coincide with the center of negative charge. This phenomenon is explained by spontaneous alignment of these permanent moments along the same direction. The term comes from the similarity with → ferromagnetism, but iron is not a ferroelectric. Ferroelectricity disappears above a critical temperature. Ferroelectric materials have been a fertile field for the study of → phase transitions.

See also: → ferro-; → electricity.

A ferroamagnetic substance, which possesses → ferromagnetism.

See also: → ferro-; → magnet.

Relative to or characterized by → ferromagnetism.

See also: → ferro-; → magnetic.

A property of certain substances which are enormously more magnetic than any other known substance. Ferromagnetic substances, such as the chemical elements iron, nickel, cobalt, some of the rare earths,
and ceratin alloys, achieve maximum → magnetization at relatively low magnetic field strengths. Their large → magnetic permeabilityies (greater than unity)
vary with the strength of the applied field. When the temperature of a ferromagnet
is increased the property vanishes gradually due to randomizing effects of thermal agitation. Beyond a definite temperature for each substance ( → Curie temperature) it ceases to behave as a ferromagnet and becomes a → paramagnet.
Ferromagnetism is due to the alignment of the → magnetic moments of uncompensated electrons in the crystal lattice. Under the influence of an external magnetizing field, all of the uncompensated electrons line up with their → spins in the direction of the field. In contrast with paramagnetic substances, in which spins interact only with an external magnetic field, in ferromagnets the spins interact with each others, each of them trying to align the others in its own direction. This coupling gives rise to a spontaneous alignment of the moments over macroscopic regions called domains. The domains undergo further alignment when the substance is subjected to an applied field. Ferromagnets retain their magnetisation even when the external magnetic field has been removed. See also → antiferromagnetism ; → diamagnetism; → magnetism.

See also: → ferro-; → magnetism.

Of or containing → iron, especially in the bivalent state. More specifically, iron with → oxidation number of +2, denoted iron(II) or Fe2+.

Etymology (EN): From L. ferrum “iron,” + -ous a suffix forming adjectives that have the general sense “possessing, full of” a given quality.

Etymology (PE): Fervar, from fer, loan from Fr., + -var adj. suffix.

Iron in a plus-2 → oxidation state.

See also: → ferrous; → iron.

Physics: (of nuclear material) Able to become → fissile by → neutron capture.

Etymology (EN): M.E., from M.Fr. fertil, from L. fertilis “bearing in abundance, fruitful, productive,” from ferre “to bear,” from PIE root *bher- “to carry,” also “to bear children,” cognate with Pers. bordan “to carry, bear,” → refer.

Etymology (PE): Bârvar, literally “fruitful,” from bâr “fruit; flower; load; charge”

• possession suffix -var, related to bordan “to bear, carry,” as above.

An → isotope not itself → fissile but that is converted into a fissile isotope, either directly or after a short → decay process following absorption of a → neutron. Example: U-238 can capture a neutron to give U-239. U-239 then decays to Np-239 which in turn decays to fissile Pu-239.

The most important fertile isotope is U-238. This is by far the most abundant isotope of natural uranium, making up 99.28%. The important transformation chain is: ₉₂U²³⁸ + ₀n¹→ ₉₃Np²³⁹ + β^- (23.5 minutes) → ₉₄Pu²³⁹ + β^- (2.36 days).

See also: → fertile; → isotope .

A schematic representation, in quantum electrodynamics and quantum chromodynamics,
of the way elementary particles like electrons and protons interact with each other by exchanging photons. Use of Feynman diagrams can greatly reduce the amount of computation involved in calculating a rate or cross section of a physical process.

See also: After the American physicist Richard P. Feynman (1918-1988), Nobel prize 1965; → diagram.

Same as → field horizontal branch star.

See also: → field; → horizontal; → branch; → star.

→ optical fiber.

Etymology (EN): From Fr. fibre, from O.Fr. fibre, from L. fibra “a fiber, filament,” of uncertain origin, perhaps related to L. filum “thread.”

Etymology (PE): Fibr, loan from Fr., as above.

One of the numbers in the → Fibonacci sequence.

See also: → Fibonacci sequence; → number.

An infinite sequence of integers, starting with 0 and 1, where each element is the sum of the two previous numbers. For example: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, … As the sequence develops, the ratio of the consecutive terms converges to the → golden ratio, about 1.618.

See also: Leonardo Pisano Fibonacci (1170-1250), medieval Italian mathematician who wrote Liber abaci (1202; Book of the Abacus), the first European work on Indian and Arabian mathematics, which introduced “Arabic” numerals in Europe; → sequence.

A linear pattern in the → chromosphere of the → Sun, as seen through an → H-alpha filter, occurring near strong → sunspots and → plages or in → filament channels. They are magnetically confined tubes of hot → plasma. Individually, they are about 10,000 km long and last for 10 to 20 minutes.

Etymology (EN): From Mod.L. fibrilla, from fibr(a) “fiber” + -illa diminutive suffix.

Etymology (PE): Târcé, from târ “thread, warp, → string”

• diminutive suffix -cé, from Mid.Pers. -cak, variants -êžak (as in kanicak “little girl,” sangcak “small stone,” xôkcak “small pig”), also Mod.Pers. -ak.

Anatomy: The outer and thinner of the two bones of the human leg, extending from the knee to the ankle.

Etymology (EN): Fom L. fibula “clasp, brooch; bolt, peg, pin,” related to figere “to drive in, insert, fasten,” → fix.

Etymology (PE): Nâzok-ney, literally “fine reed,” from nâzok “→ fine” + ney “reed, pipe, flute.”

1. Literary works invented by the imagination, such as novels or short stories.
   

2. An invented story or explanation; lie.
   

3. The act of inventing a story or explanation.
   

4. Law: Something assumed to be true for the sake of convenience, though probably false (Dictionary.com).

Etymology (EN): M.E., from O.Fr. ficcion “dissimulation, ruse; invention, fabrication” and directly from L. fictionem “a fashioning or feigning,” noun of action from p.p. stem of fingere “to shape, form, devise, feign,” originally “to knead, form out of clay,” from PIE *dheigh- “to build, form, knead;” akin to Skt. dehah “body,” literally “that which is formed,” dih- “to besmear;” Gk. teikhos “wall;” L. fingere “to form, fashion,” Gothic deigan “to smear;” O.Irish digen “firm, solid.”

Etymology (PE): Formed on the model of fiction, as above, from diz- “to build, to form;” (related to Pers. dež, dez “fortress”);
cf. Mid.Pers. dys-/dēs- “to build;” Sogd. dys “to build;” Av. (+ *pari-) daēz- “to build (around);” Proto-Ir. *daiz- “to build, form;” from PIE *dheigh- “to build, form,” as above, + suffix -an.

1. General: An expanse of anything.
   

2. Physics: A region or space defined by the presence of a physical force, such as electric, magnetic, or gravitational.
   

3. Math.: A mathematical entity for which addition, subtraction, multiplication, and division are well-defined.

Etymology (EN): M.E., from O.E. feld “plain, open land,” probably related to O.E. folde “earth, land,” from P.Gmc. *felthuz “flat land” (cf. Ger. Feld), from PIE *pel(e)-tu-,from base *pele- “flat, to sprea;” cf. L. planus “flat, level,” → plane.

Etymology (PE): Meydân “field, arena, extensive plain; town square; gathering place” from Mid.Pers. mêdân “arena, field.” Meydân has been borrowed into various languages: Ar. maydân, Turkish meydan, Crimean Tatar, Ukrainian maidan, Polish maidan, also in Pakistan, India, Indonesia, Nepal.

An aberration in an optical instrument, common in Schmidt telescopes, in which the focus changes from the center to the edge of the field of view. Owing to this aberration, a straight object looks curved in the image.

See also: → field; → curvature.

In a physical theory, an equation that describe how a fundamental force interacts with matter. Einstein’s equations of → general relativity are called field equations since they describe the → gravitational field. Similarly, → Maxwell’s equations describe the electromagnetic field.

See also: → field; → equation.

A galaxy that lies in the direction of a → cluster of galaxies, but is not a member of the cluster. Field galaxies are rare, less than about 5% of all galaxies.

See also: → field; → galaxy.

A → horizontal branch star with high velocity.

See also: → field; → horizontal; → branch; → star.

A lens placed at or near the focal plane of a telescope to create an image of the primary mirror inside the instrument.

See also: → field; → lens.

An → O-type star which is apparently not associated with a → star cluster. A significant fraction of → massive stars in the → Milky Way and other galaxies are located far from star clusters and → star-forming regions. It is known that some of these stars are → runaways, i.e. possess high → space velocities (determined through the → proper motion and/or → radial velocity measurements), and therefore most likely were formed in embedded clusters and then ejected into the field because of dynamical interactions or → binary-→ supernova explosions. However, there exists a group of field O stars whose runaway status is difficult to prove via direct proper motion measurements or whose low space velocities and/or young ages appear to be incompatible with their large separation from known star clusters. The existence of this group led some authors to believe that these stars can form → in situ. The question of whether or not O stars can form in isolation (→ isolated massive star formation) is of crucial importance for → star formation models (Gvaramadze et al., 2012, MNRAS, 424, 3037).

See also: → field; → O star.

An astronomical object that is seen in the direction of a group but not physically belonging to the group. → field galaxy.

See also: → field; → object.

The region of → space surrounding a body, such as a mass of → matter, a → charged particle, or a → magnet, within which it can exert a → force on another similar body not in contact with it.

See also: → field; → force.

The entire angular expanse of the sky viewed by an optical instrument.

See also: → field; → view.

The effect of the Earth’s rotation on the position of the image formed on the → focal plane of a telescope during long exposures. In the case of → equatorial mounting, the image remains fixed, whereas it turns continuously with an → altazimuth mounting. In the latter case the image motion must be compensated by an appropriate mechanism, → field rotator.

See also: → field; → rotation.

A device used on a telescope to correct for the → field rotation while tracking an object.

See also: → field; → rotator.

A star that does not belong to a stellar cluster, but happens to be adjacent to it.

See also: → field; → star.

A diaphragm located at an image plane of an optical system that determines the size and shape of the image. → aperture stop.

See also: → field; → stop.

1. A theory which uses the concept of → field to describe physical phenomena. It consists of two types:
   → classical field theory and → quantum field theory.
   

2. Math.: A branch of mathematics which studies the → fields.

See also: → field; → theory.

The precise curve required on the surface of an optical element, especially the mirror of a reflecting telescope.

Etymology (EN): From O.Fr. figure, from L. figura “a shape, form.”

Etymology (PE): Šekl from Ar. šakl “figure.”

The final stage of mirror making, a process of altering the mirror’s shape into the one required for proper function. → grinding;
→ polishing.

Etymology (EN): Figuring, noun of → figure.

Etymology (PE): Šekldahi, “giving form,” from šekl, → figure + dahi, a verbal noun of dâdan “to give,” from
Mid.Pers. dâdan “to give,” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives;” cf. Skt. dadáti “he gives,” Gk. didomi “I give,” tithenai “to put, set, place;” L. dare “to give, offer;” facere “to do, to make;” Rus. delat “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do.”
PIE base *dhe- “to put, to do.”

1. A long tongue of a relatively cold matter (10,000 K), suspended in the → solar corona (2 million K). Filaments seem dark in the Hα light when they are seen projected on the solar disk; at the limb they look as what they actually are: → prominences.
   

2. Electricity, Electronics: In electric light bulbs and thermionic valves, a wire of metal of high melting point, which is heated by the passage of an electric current.
   

3. Cosmology: → cosmic filament.

Etymology (EN): From Mod.L. filamentum, from L.L. filare “to spin, draw out in a long line,” from L. filum “thread,” from PIE base *g^whi- “thread, tendon;” cf. Gk. bios “bow;” Skt. jiyā- “bow-string;” Av. jiiā- “bow-string;” Mod.Pers. zeh “string, bow-string” (dialectal Qomi zij “mason’s cord”);
Arm. jil “string, line;” Lith. gijà “thread;” Russ. žica “thread.”

Etymology (PE): Rešté “thread, line, file,” from reštan, ristan “to spin;” Mid.Pers. ‘rws- “to turn to,” abrešom “silk;” Sogdian rwyšt “spun;” Av. uruuaēs- “to twist, turn;” Proto-Iranian *uris- “to turn, spin.”

A nebula, generally ionized, consisting of filament-like structures of gas, such as the
Veil Nebula (NGC 6960) or the supernova remnant IC 443.

See also: Filamentary adj. of → filament; → nebula.

An instrument used with a telescope for accurately measuring small angular separations between two celestial bodies (as between binary
stars). The instrument consists of two parallel fine wires with one wire being fixed and the other movable by means of a finely threaded screw.

Etymology (EN): Filar, from L. fil(um) “a thread” (see the paragraph below)

• -ar variant of the adjective-forming suffix → -al;
  → micrometer.

Etymology (PE): Kehsanj, → micrometer; zehi adj. of zeh “string, bow-string;” Mid.Pers. zih “bow-string,” zig “string; astronomical table” (loaned into Ar. as zij);
Av. jiiā- “bow-string;” cf. Skt. jiyā- “bow-string;” Gk. bios “bow;” L. filum “thread;” Arm. jil “string, line;” Lith. gijà “thread;” Russ. žica “thread;” PIE base *g^whi- “thread, tendon.”

In computer science, a collection of related data or program records stored on a support.

Etymology (EN): From M.E. filen, from M.F. filer “to string documents on a thread,” O.F. filer “to wind or spin thread,” from L.L. filare “to spin, draw out in a long line,” from L. filum “thread,” from PIE base *g^whi- “thread, tendon;” cf. Gk. bios “bow;” Skt. jiyā- “bow-string;” Av. jiiā- “bow-string;” Mod.Pers. zeh “bow-string;” Arm. jil “string, line,” Lith. gijà “thread;” Russ. žica “thread.”

Etymology (PE): Parvandé “file,” initially “a bundle, roll, truss (of clothes),” from Mid.Pers. parwastan, parwand- “to surround, enclose, contain, comprise,” from par-, variants far-, par-, pirâ- “around, about” (Mid.Pers. pêrâ; O.Pers. pariy “around, about,” Av. pairi “around, over;” Skt. pari; Indo-Iranian *pari- “around;” PIE base *per- “through, across, beyond;” cf. Gk. peri “around, about, beyond;” L. per “through”) + vand-/band- stem of vastan/bastan “to bind, shut;” O.Pers./Av. band- “to bind, fetter,” banda- “band, tie” (cf.
Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind;” Ger. binden; E. bind).

Of a molecular cloud or a nebula, the ratio of the volumes filled with matter to the total volume of the cloud.

Etymology (EN): Filling, from fill, from O.E. fyllan, from P.Gmc. *fullijan (cf. Du. vullen, Ger. füllen “to fill”), a derivative of adj. *fullaz→ full; → factor.

Etymology (PE): Karvand, → factor; pori, from por,
→ full.

1. General: A device for separating the constituents of a system according to a physical property. In particular, a device for removing solid matter suspended in a liquid by forcing the suspension through a material which retains the solid matter while allowing the liquid to pass.
   

2. Optics: A device for transmitting electromagnetic radiation with restricted ranges of wavelength. Two types are generally used, → glass filters and → interference filters. See also → neutral density filter.
   

3. Electronics: A circuit or equipment allowing the transmission of desired frequencies and the elimination of others.

Etymology (EN): From M.L. filtrum “felt,” which was used to strain impurities from liquid, from W.Gmc. *filtiz.

Etymology (PE): Pâlâyé, from pâlây present stem of pâludan “to filter, purify;” Mid.Pers. pâludan, pâlây- “to strain, filter, purify,” from Proto-Iranian *pari-harz-, from prefix *pari- “around,” → peri-,

• harz- “to let go, release;” cf. Av. pairi.harəz- “to filter,” from pairi- “around, over” (cf. Skt. pari; Gk. peri “around, about, beyond;” L. per “through;” PIE base *per- “through, across, beyond”)
• harəz- “to release” (especially water, liquid; cf. Chorasmian pžy- “to purify, filter;” Arm. (loanword) parzel “to filter, purify;”

Pertaining to or coming at the end; last in place, order, or time.

Etymology (EN): M.E., from O.Fr. final from L. finalis “of or pertaining to an end, concluding,” from finis “end.”

Etymology (PE): Pâyâni from pâyân “end, extremity; limit, boundary,” from pâ(y) “foot; step; track,” → foot.

A meteorite that was not seen to fall, but was found at some later date, as opposed to a → fall.

Etymology (EN): O.E. findan “to come upon,” from P.Gmc. *finthanan (cf. M.Du. vinden, Ger. finden), from PIE *pent- “to go, pass, path, bridge;” cf. Av. paθ-, variants paθi-, paθā-, pantay-; Mid/Mod.Pers. pand “path, advice, councel;” Khotanese pande “road, path;” Ossetic fœndœg “path, road;” cf. Skt. pánthā- “road, path, course;” Gk. patos “path, way,” pontos “sea;” L. pons “bridge, path.”

Etymology (PE): Yâft, past stem of yâftan, yâbidan “to find, discover; to obtain, acquire;” Mid.Pers. ayâftan, ayâpênitan “to reach, attain;” Manichean Mid.Pers. ‘y’b “to attain;” Parthian, Sogdian (+ *pati-) pty’b “to reach, obtain;” Av. ap- “to reach, overtake,” apayeiti “achieved, reached;” Skt. âp- “to reach, gain,”
âpnoti “reaches, gains;” Gk. hapto, haptomai “to touch, cling to, adhere to;” L. apiscor “touch, reach;” PIE base *ap- “to take, reach.”

A low-power telescope with a wide field of view attached to a larger telescope with the optical axes of both telescopes parallel. The finder is used to help point the larger telescope to the desired viewing location.

See also: Agent noun of → find.

A sketch or image used to recognize objects in the field of view of a telescope.

Etymology (EN): Finding, noun of → find; chart,
from M.Fr. charte “card, map,” from L. charta
“leaf of paper, tablet,” from Gk. khartes “layer of papyrus.”

Etymology (PE): Negâre-ye yâbeš, from negâré, from negâr “picture, figure” (verb negârdan, negâštan “to paint”), from prefix ne- “down; into,” → ni-, + gâr, from kar-, kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”); yâbeš, verbal noun of yâftan, → find.

Very thin or slender. → fine structure, → fine-structure constant.

Etymology (EN): M.E. fin, from O.Fr. fin “perfected, of highest quality,” from L. finis “end, limit.”

Etymology (PE): Nâzok “thin, slender, subtle,” from Mid.Pers. nâzuk “tender, gentle,” variant nâzik, from nâz “joy, pride, glory” + → -ik.

Meteorology: An → inhomogeneous  → mixture of tiny, part → solid, part → liquid or → gaseous  → particles that are, in average, smaller than ten → microns. The constituents are soot, heavy metals, organic substances, and dioxins. The smaller these dust particles, the deeper they penetrate into the lung. Larger particles are intercepted by mucous membrane in nose, mouth, and throat but smaller particles can penetrate the smallest lung bronchioles and may cause severe damage (various respiratory disorders, lung cancer) → particulate matter.

See also: → fine; → dust.

Closely spaced components seen at high resolution in a → spectral line. The phenomenon is explained by the fact that instead of a single → energy level corresponding to a given value of the → quantum number  n, there are actually a number of energy levels lying close to one another. → fine-structure constant, → fine-structure line.

See also: → fine; → structure.

A measure of the strength of → interaction between a → charged particle and the → electromagnetic field. It is a → dimensionless number expressed (in → cgs units) by α = e²/ħc, where e is the → electron charge, ħ is the → reduced Planck’s constant, and c is the → speed of light. It is approximately equal to 1/137 or 7.3 × 10^-3.
The smallness of this number is of great importance since it determines the size of → atoms and the → stability of → matter. Same as → electromagnetic coupling constant.

See also: → fine structure; → constant.

A → spectral line whose → energy levels have a → fine structure. Examples are [C II] 157.7 μm (→ singly ionized carbon), [O III] 88 μm, and [Ne II] 12.8 μm.

See also: → fine; → structure; → line.

Any of the terminal members of the hand, especially one other than the thumb (Dictionary.com).

Etymology (EN): M.E., from O.E. fingor, cognate with Ger. Finger, Du. vinger,
O.N. fingr, Goth. figgrs.

Etymology (PE): Angošt, variants angol, angul (also angal “loop”);
Mid.Pers. angust; Av. angušta- “toe,”
ank- “curved, crooked;” cf. Skt. angustha- “thumb,” angula- “finger,” ankah “hook, bent;” Gk. angkon “elbow,” angkura “anchor;” L. angulum “corner;” Lith. anka “loop;” O.E. ancleo “ankle;” O.H.G. ango “hook;” PIE *ang-/*ank- “to bend”.

A weak yet important kind of mixing that results from → fingering instability in stars within → radiative zones that have an unstable mean → molecular weight  → gradient. Also called
→ thermohaline convection.

See also: → finger; → -ing; → convection.

A type of instability that often occurs in fluids which are thermally stably stratified, but have an inhomogeneous composition. A well-known example, found in upper layers of the Earth’s oceans, is
→ salt fingers. Similar fingering instabilities can occur in any other thermally stably stratified solution, provided the concentration of the slower-diffusing solute increases with height. The saturated state of this instability, → fingering convection, takes the form of tightly-packed, vertically-elongated plumes of sinking dense fluid and rising light fluid, and significantly enhances the vertical transport of both heat and chemical composition. The fingering instability occurs in stars within radiation zones that have an unstable mean → molecular weight  → gradient (μ gradient). This situation is often found as a result of material accretion onto a star by anything from a single or multiple planets, to material from a dust-enriched or debris accretion disk, or material from a more evolved companion. It also naturally arises in the vicinity of the → hydrogen shell burning in → red giant branch (RGB) stars, and in thin element-rich layers near the surface of intermediate-mass stars. The fingering instability initially takes the form of thin tubes, hence the name “finger,” within which the fluid moves vertically. The tubes rapidly break down, however, as a result of parasitic shear instabilities that develop inbetween them, and the fingering instability eventually saturates into a state of homogeneous fingering convection where the typical aspect ratio of the eddies is closer to one (P. Garaud et al., 2015, arXiv:1505.07759).

See also: → finger; → -ing; → instability.

A → redshift space distortion which causes the dense central regions of → galaxy clusters to appear elongated along the → line of sight. This effect is attributed to random velocities in clusters of galaxies deviating from pure → Hubble flow. For an observer galaxies with peculiar velocity perpendicular to the line of sight do not change the redshift, which is given just by the normal Hubble expansion. On the contrary, galaxies with peculiar velocity along the line of sight appear with a different redshift, resulting from the Hubble expansion velocity plus the peculiar velocity. Since this affects only redshift and not position on the sky, the stretching occurs only radially, toward the observer. See also → Kaiser effect, → peculiar velocity.

See also: → finger; → God.

1. Math: The opposite of → infinite.
   

2. Physics: Either non-infinite or non-zero.

Etymology (EN): From L. finitus, p.p. of finire “to limit, set bounds, end.”

Etymology (PE): Karânmand, from karân “boundary, side, end, coast” + -mand adjective suffix. Karân, variants karâné, kenâr, from Mid.Pers. karân, karânak, kenâr “edge, limit, boundary,” Av. karana- “side, boundary, end.”

A → statistical population consisting of individuals or items which are finite in number.

See also: → finite; → population.

A sum a₁ + a₂ + a₃ + · · · + a_N, where the a_i’s are real numbers. In terms of Σ-notation, it is written as

a₁ + a₂ + a₃ + · · · + a_N = Σ (n = 1 to N).  See also → infinite series.

See also: → finite; → series.

A → set whose elements can be numbered from 1 to n, for some positive integer n.

See also: → finite; → set.

A state, process, or instance of combustion in which a substance combines with oxygen producing heat, light, and flame.

Etymology (EN): O..E. fyr, from P.Gmc. *fuir (cf. O.N. fürr, M.Du. vuur, Ger. Feuer), from PIE *paewr-; cf. Mod.Pers. Lori porpor “blazing charcoal,” Gilaki bur, biur “smokeless red fire” (Lori perisk, periska “spark,” Kurd. biriske “spark,” Lârestâni pelita “spark”); Tokharian por, puwar “fire;” Gk. pyr “fire;” Hitt. pahhur “fire;” Skt. pū- “to cleanse.”

Etymology (PE): Âtaš, variants âzar, taš, from Mid.Pers. âtaxš, âtur “fire;” Av. ātar-, āθr- “fire,” singular nominative ātarš-; O.Pers. ātar- “fire;” Av. āθaurvan- “fire priest;” Skt. átharvan- “fire priest;” cf. L. ater “black” (“blackened by fire”); Arm. airem “burns;” Serb. vatra “fire;” PIE base *āter- “fire.”

A → meteor that is brighter than the brightest planets, i.e. with an apparent magnitude of -5 or greater. Fireballs are often followed by → meteorite falls. Also called → bolide.

Etymology (EN): From → fire + ball, from O.E., from O.N. bollr “ball,” from P.Gmc. *balluz (cf. O.H.G. ballo, Ger. Ball), from PIE base *bhel- “to swell.”

Etymology (PE): Tašguy, from taš “fire,” variant of âtaš→ fire + guy “ball, sphere,” variants golulé, gullé, goruk, gulu, gudé (cf. Skt. guda- “ball, mouthful, lump, tumour,” Pali gula- “ball,”
Gk. gloutos “rump,” L. glomus “ball,” globus “globe,” Ger. Kugel, E. clot; PIE *gel- “to make into a ball”).

1. Having a solid structure that resists pressure.
   

   2. Solidly or securely fixed in place.

Etymology (EN): M.E. ferm, from O.Fr. ferm “strong, vigorous; healthy, sound; steadfast,” from L. firmus “strong, steadfast, stable,” from PIE root *dher- “to hold firmly, support.”

Etymology (PE): Deš (Hamadâni) “firm, steady,” (Ilâmi) dež “intact, entire,” (Hamadâni) daj “intact, full,” (Šuštari) dec “full,” may be related to Proto-Ir. *dar- “to hold, keep;” Av. dār- “to hold, keep;” Pers./ Mid.Pers. dâštan/dâr- “to have, hold, keep;” cf. Skt. dhar- “to hold, keep, preserve;” L. firmus “firm, steady, as above.”

Being before all others with respect to time, order, importance, etc., used as the ordinal number of one.

Etymology (EN): O.E. fyrst “foremost,” superlative of fore, from P.Gmc. *furisto (cf. O.H.G. furist, O.N. fyrstr, Dan. første, M.Du. vorste “first,” Ger. Fürst “prince”), superlative of *fur-/*for-, from PIE *pro- (cf. Av. pouruua- “first,” fra- “forward, forth;” Skt. pūrva- “first,” pra- “before, formerly,” Gk. pro; L. pro; E. fore).

Etymology (PE): Naxost, from Mid.Pers. naxust “the first,” Parthian Mid.Pers. nxwšt, from naxu, Manichean Parthian nwx “beginning” + -ist superlative suffix, Av. -išta-, cf. Skt. -istha-, Gk. -istos, O.H.G. -isto, -osto, O.E. -st, -est, -ost; naxostin, from naxost + suffix -in.
Yekom, from yek, → one, + -om suffix of ordinal numbers.
Âqâz “beginning,” from Proto-Iranian *āgāza-, from prefix ā-

• *gāz- “to take, receive,” cf. Sogdian āγāz “beginning, start,” pcγz “reception, taking.”

1. Generally, an expression to indicate that a comment or result is only approximate.
   

2. Math.: In calculus, limiting a differential equation to its → first derivative, for example: e^x≅ 1 + x. Also called → linear approximation.

See also: → first; → approximation.

An early phase in the process of star formation which begins when the mass of a → molecular cloud → clump exceeds the → Jeans mass.
The collapse is initially → optically thin to the thermal emission from → dust grains, and the compressional heating rate is much smaller than the cooling rate by the → thermal radiation. The collapse proceeds
→ isothermally. The isothermal condition is broken when the central density reaches about 10^-13 g cm^-3 and a small region at the center of the cloud starts to become → opaque. The heat generated by the collapse in this region is no longer freely radiated away, and the compression becomes approximately → adiabatic. The central temperature and pressure then begin to rise rapidly, soon becoming sufficient to decelerate and stop the collapse at the center.
There then arises a small central core, called the → first core, in which the material has stopped collapsing and is approaching → hydrostatic equilibrium.
Outside this core, the material is still nearly isothermal and continues to fall inward almost in → free fall.
Consequently a shock front arises at the boundary of the core, where the infalling material is suddenly stopped. The initial mass and radius of the core are about 10³¹ g and 6 x 10¹³ cm, respectively, and the central density and temperature are about 2 x 10^-10 g cm^-3 and 170 K, respectively. As the collapse proceeds, the core grows in mass due to the infall of the surrounding material; at the same time, however, the core radius decreases because of radiative energy losses from the outer layers of the core. The process leads to the → second collapse (R. B. Larson, 1969, MNRAS 145, 271).

See also: → first; → collapse.

1. The beginning of a → solar eclipse when the eastern part of the lunar limb touches the western limb of the Sun, marking the beginning of an eclipse.
   

2. For a → lunar eclipse, the moment when the eastern limb of the Moon is the first to enter the Earth’s shadow.

See also: → first; → contact.

A first object in → hydrostatic equilibrium predicted to form during early dynamical contraction of a → molecular cloud → clump in the course of the → first collapse.

See also: → first; → core.

A equation in which the highest → exponent of the → variable is 1. Same as
→ linear equation.

See also: → first; → degree; → equation.

The simplest mathematical → differentiation of one quantity relative to another; df(x)/dx. It is the same as → derivative, contrasted with the → second derivative or the nth derivative.

See also: → first; → derivative.

In → calculus, a method for determining whether an → inflection point is a → local minimum, → local maximum, or neither.

See also: → first; → derivative; → test.

The → dredge-up occurring after core hydrogen burning as the core contracts before helium burning ignites (on the ascending giant branch). The hydrogen envelope becomes convective and this convective zone penetrates deep into the core dredging up material that has been processed by the central nuclear reactions. As a result the abundances of helium and nitrogen are boosted.

See also: → first; → dredge-up.

The total energy of a → closed system is constant. This means that energy can be changed from one form to another, or transferred from one system to another, but it cannot be created or destroyed. A mathematical formulation of the first law is: δQ = δU + δW, where δQ is the heat transferred to the system, δU the change in internal energy (resulting in a rise or fall of temperature), and δW is the work done by the system.

See also: → first; → law; → thermodynamics.

The first astronomical observation done with a major newly built telescope.

See also: → first; → light.

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → vernal equinox several thousands years ago.

See also: → first; → point; → Aries.

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → autumnal equinox several thousands years ago. Because of → precession, this equinoctial point no longer lies in Libra but in neighboring Virgo.

A → lunar phase that occurs in the middle of the interval between the → new Moon and the → full Moon, when half of the Moon’s disk is illuminated. At first quarter, the Moon is situated at 90° east of the Sun.

See also: → first; → quarter.

A member of the → Population III → massive stars that formed some 500 Myr after the → Big Bang. First stars had a simple chemical composition consisting only of H, He, and traces of ⁷Li and were very short-lived. They are the most likely sources of → reionization of the Universe,
which put an end to the → Cosmic Dark Age.

See also: → first; → star.

A → differential equation containing only the first → derivative. For example, dy/dx = 3x and 2y(dy/dx) + 3x = 5.

See also: → first; → order; → differential; → equation.

A system of → formal logic that is an extension of → propositional logic. It is also known as → first-order predicate calculus and → predicate logic. FOL eliminates deficiencies of propositional logic by: representing → objects (their properties, relations and statements about them), introducing → variables, and introducing → quantifiers.

See also: → first; → order; → predicate; → logic.

For a → function with several → variables, the derivative, done only once, with respect to one of the variables. For example ∂u/∂x for u = u(x,y).

See also: → first; → order; → partial; → derivative.

Same as → first-order logic.

See also: → first; → order; → predicate; → calculus.

An optical spectrum, produced by a diffraction grating, in which the difference in path length of light from adjacent slits is one wavelength.

See also: → first; → order;
→ spectrum.

A limbless cold-blooded vertebrate animal with gills and fins living wholly in water (OxfordDictionaries.com).

Etymology (EN): M.E. fis(c)h, fyssh, O.E. fisc; cognate with Du. vis, Ger. Fisch, O.Norse fiskr, Goth. fisks; akin to L. piscis; PIE root *pisk- “a fish.”

Etymology (PE): Mâhi “fish,” from Mid.Pers. mâhik; Av. masya-; cf. Skt. matsya-, Pali maccha-.

Any material that is capable of undergoing → nuclear fission by → thermal neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239. Although sometimes used as a synonym for → fissionable material, this term has acquired a more restricted meaning.

Etymology (EN): From L. fissilis, from fiss(us), + -illis a suffix of adjectives expressing capability, susceptibility, liability, aptitude, etc.

Etymology (PE): Šekâftani, from šekâft, → fission + -i a suffix expressing capability, aptitude, etc.

An isotope that is capable of undergoing nuclear fission after capturing either fast neutron or thermal neutron. Typical fissionable isotopes: ²³⁸U, ²⁴⁰Pu, but also ²³⁵U, ²³³U, ²³⁹Pu, ²⁴¹Pu

See also: → fissile; → isotope .

1. The act or process of splitting or breaking into parts.
   

2. Splitting of the nucleus of an atom into two or more fragments of comparable size, usually as the result of the impact of a neutron on the nucleus. Same as → nuclear fission.

Etymology (EN): Fission, from L. fissionem “a breaking up, cleaving,” from root of findere “to split.”

Etymology (PE): Šekâft, stem of šekâftan “to split, break, tear,” akin to kaftan, kâftan “to split; to dig,” Parthian Mid.Pers. q’f- “to split;” Sogdian kβ “to split;” Chorasmian kf- “to split, be split;” Proto-Iranian *kap-, *kaf- “to split.”

Nuclides generated by the fission of higher mass elements or by subsequent radioactive decay of nuclides directly generated by fission.

See also: → fission; → product.

A theory that suggests the Moon was formed at the same time as Earth. A spinning Earth ejected a large piece of its material into space which then developed into the shape and orbit of the Moon. This event was also thought to be at the origin of the Pacific Ocean.

This first modern idea about the formation of the Moon is due to George Darwin, the son of the great naturalist Charles Darwin.

The fission theory explained the lack of volatile substances on the Earth. The volatile materials on the Earth would have been thrown out into space

The fission theory is almost completely abandoned today. The analysis of lunar rocks brought to Earth by NASA astronauts showed that the Moon rocks are older than the rocks at the bottom of the Pacific Ocean. Moreover, modern → plate tectonics gives a better explanation of the origin of the Pacific Ocean. See also → giant impact hypothesis, → capture theory, → co-formation theory.

See also: → fission; → theory.

The material that can be fissioned by fast neutrons, such as uranium-238. Commonly used as a synonym for → fissile material.

See also: From → fission + → -able.

1. (v.) To pass a mathematical line or curve through data points.
   
2. (n.) The act of fitting.

Etymology (EN): From M.E. fitten; akin to M.Du. vitten “to befit.”

Etymology (PE): Saz, from sazidan “to suit, fit, be worthy,” sazâ “suitable, agreeing with, congruous, deserving of,” Mid.Pers. sacitan/sazidan “to fit,” sazešn “fitness,” sazâg “fitting, worth;” Av. ¹sak- “to understand or know a thing; to mark;” cf. Skt. śak- “to be able, powerful” śakta- “able, competent,” śakti- “ability, power;” alternatively from Av. ²sak- “to go by, pass, pass away; to be up or over (of time).”

The process or instance of adapting a mathematical curve to data points.

See also: Verbal form of → fit.

The discrepancy between the mathematical curve and data points. → fit.

See also: → fitting; → error.

A cardinal number whose symbol is 5, V, or ۵.

Etymology (EN): Five, from
O.E. fif, from P.Gmc. *fimfe (cf. O.S. fif, O.H.G. funf), from PIE *penk^we “five;” cognate with Pers. panj, as below.

Etymology (PE): Panj, from Mid.Pers. panj; Av. panca; cf.
Skt. páñca; Gk. pente; L. quinque; E. five, as above.

A photometric system which uses five filters, from ultraviolet to the red part of the visual spectrum: U, B, V, R and I.

See also: → five; → color; → system.

The 500 m diameter → radio telescope which is the largest → single-dish antenna in the world. It is an Arecibo type telescope nestled within a natural basin in China’s remote and mountainous Dawodang, Kedu Town, in southeastern China’s Guizhou Province. The → reflector consists of 4,450 triangular panels, each with a side length of 11 m. More than 2,000 → actuators are used, according to the feedback from the measuring system, to deform the whole reflector surface and directly correct for → spherical aberration. Several detectors are used to cover a frequency range of 70 MHz to 3 GHz.

See also: → five; → hundred; → meter; → aperture; → spherical; → radio; → telescope.

1. To make fast, firm, or stable.
   

2. To place definitely and more or less permanently (Dictionary.com).

Etymology (EN): M.E. fixen, probably from O.Fr. fixe “fixed,” from L. fixus “fixed, fast, established, settled,” p.p. of figere “to fix, fasten.”

Etymology (PE): Infinitive, from barjâ, → fixed.

The act of fixing or the state of being fixed.

See also: Verbal noun of → fix; → -tion.

1. Fastened, attached, or placed so as to be firm and not readily movable; firmly implanted; stationary; rigid.
   

2. Rendered stable or permanent, as color.
   

3. Set or intent upon something; steadily directed (Dictionary.com).

Etymology (EN): Past participle from → fix.

Etymology (PE): Barjâ “fixed; in place; properly placed,” from bar “on, upon, up,” → object,

• jâ, → place,

A → heavenly body that, in → contrast to the → planets, does not appear to change its
relative → position on the → celestial sphere.

Etymology (EN): Fixed, p.p. of → fix; → star.

Etymology (PE): Setâré, → star; barjâ, → fixed.
Istâdé (Biruni in at-Tafhim) “standing,” from istâdan “to stand;” O.Pers./Av. sta- “to stand, stand still; set” (cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh;” stasis “a standing still;” Lith. statau “place;” Goth. standan; L. stare “to stand;” PIE base *sta- “to stand”);
Biyâbâni (Biruni in at-Tafhim), from Mid.Pers. awiyâbânig “fixed,” from negation prefix → a- + wiyâbânig “wandering,” from wiyâbân “deluded, seduced,” wiyâbânênidan “to lead astray, deceive,” from Av. vi- “apart, away from, out” (O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”)

• dab- “to deceive;” cf. Skt. vimugdha- “confused, bewildered,” vimohita- “confused, infatuated.” Note that biyâbân “desert” is from an other origin.

The determination of the shape of an optical surface by means of → interferometry using a → fringe pattern formed with respect to a reference surface.

See also: Named after the French physicist Armand Hippolyte Louis Fizeau (1819-1896), see also → toothed-wheel experiment; → test.

1. Burning gas or vapor, as from wood or coal, that is undergoing combustion; a portion of ignited gas or vapor.
   

2. Any flame-like condition; glow; inflamed condition (Dictionary.com).

Etymology (EN): M.E. flaume, variant of flaumbe from Anglo-French flaume, flaumbe “a flame;” O.Fr. flambe, from L. flammula “small flame,” diminutive of flamma “flame, blazing fire,” from PIE *bhleg- “to shine, flash,” from root *bhel- (1) “to shine, flash, burn.”

Etymology (PE): Âyež, from (Dehxodâ) âyež “flame,” variants âyiž, âyežé, ižak, of unknown origin.

A stellar designation system in which each star is assigned a number followed by the Latin genitive of its corresponding → constellation, such as → 61 Cygni and 82 Eridani. Compare with the → Bayer designation.

See also: Named after John Flamsteed (1646-1719), founder of the Greenwich Observatory, and the first astronomer royal of England, who introduced this system in his catalog Historia Coelestis Britannica (1725); → designation.

1. A sudden blaze or burst of fire or light.
   

2. → solar flare.

Etymology (EN): From v. flare “to spread out,” said of hair, a ship’s sides, etc., of unknown origin.

Etymology (PE): Âlâv, “blaze, fire,” variants alow, Borujerdi elew “fire,” Garkuyeyi alôv “flame,” Hamadâni elow “flame,” Lori alô “flame,” Tabari aluk “flame, spark,” Torbat-Heydariyeyi alow “flame;”
cf. Gk. aithos “fire,” aitho “to kindle;” Skt. edh- “to set alight, kindle,” édha-, édhas- “firewood;” Av. aēsma- “firewood;” Mod.Pers. hizom, himé “firewood;” PIE base *aidh- “to burn,” *aidhos- “fire.”

A member of a class of dwarf stars that undergoes sudden, intense outbursts of light (mean amplitude about 0.5-0.6 mag).

See also: → flare; → star.

A model of → accretion disk around a → pre-main sequence star or a → protostar in which the ratio of the disk thickness to the distance from the star increases outward. Current models of the irradiation of flared disks by stellar radiation predict that a central hole is created around the young star due to the evaporation of dust by the stellar radiation. The inner rim of the disk, at 0.5 to 1 AU from the star, is irradiated by the star “frontally” (at 90° angle). The heat produced by the irradiation causes the inner rim to puff up. A part of the disk, from about 1 to 6 AU, lies in the shadow of the puffed-up inner rim. The surface layers in this region do not receive stellar photons directly. Therefore, there is no significant heating of the disk midplane by reprocessed stellar flux from the disk surface. The midplane temperatures in the shadowed part of the disk are governed by the → near infrared emission of the inner rim, scattering of stellar light by dust particles outside the disk plane, and radial diffusion which exchanges energy between adjacent slabs.
As for the outer parts of the disk, the surface is irradiated by the central star thanks to the outward widening of the disk. These parts remain flared, because the absorbed stellar flux is partially emitted toward the midplane, keeping the internal temperatures high enough to push the surface layers up. The flattened-disk model explains the observed
→ spectral energy distribution of some objects
such as HD 179218. It also accounts for the observed strong → far-infrared, → excess, strong → PAH emission, and strong [O I] emission. Compare with → self-shadowed disk. See also → protoplanetary disk.

Etymology (EN): Flared, from flare “to spread gradually outward, as the end of a trumpet, having a gradual increase in width,” of unknown origin; → disk.

Etymology (PE): Gerdé, → disk; borun-gošâ “opening outward,” from borun “out, the outside” (Mid.Pers. bêron, from bê “outside, out, away” + rôn “side, direction;” Av. ravan- “(course of a) river”) + gošâ stem of gošâdan, gošudan “to open;” Mid.Pers. wišâdan “to open, let free;” Khotanese hiyā “bound;” O.Pers. višta “untied, loosend;” vištāspa- (personal name) “with loosened horses;” Av. višta “untied,” hita- “fastened, tied on;” cf. Skt. sā- “to bind, fasten,” syáti “binds.”

A Sudden, brief burst of light. In particular, → green flash.

Etymology (EN): From M.E. flasshen “to sprinkle, splash,” from flasken, probably imitative.

Etymology (PE): Deraxš, present stem of deraxšidan “to shine, radiate,” from raxš “lightening, reflection of light,” raxšidan “to shine, flash,” O.Pers. raucah-, Av. raocah- “light” (cf. Skt. roka- “brightness, light,” Gk. leukos “white, clear,” L. lux “light” (also lumen, luna), E. light, Ger. Licht, Fr. lumière;
PIE base *leuk- “light, brightness”); cognate with Mod.Pers. words
ruz “day,” rowšan “bright, clear,” foruq “light,” and afruxtan “to light, kindle.”

The spectrum of the solar → chromosphere obtained during a → solar eclipse in the instant before or after → totality. In the flash
spectrum the usual solar → absorption lines are replaced with
bright → emission lines. This is because in that very short interval only the → photosphere is eclipsed by the Moon, and not the chromosphere. The American astronomer Charles A. Young was the first to observe it during the the solar eclipse of 1870 (December 22) in Spain.

See also: The flash designation comes from the fact that the change from dark-line to bright-line spectrum is very rapid; → flash; → spectrum.

Level and horizontal, without any slope; even and smooth, without any bumps or hollows.
See also:
→ flat manifold, → flat Universe, → flat-field, → flatness problem.

Etymology (EN): Flat, from O.N. flatr, from P.Gmc. *flataz (cf. O.H.G. flaz “flat, level,” O.E. flet, O.H.G. flezzi “floor”), perhaps from PIE *pla- (cf. Gk. platys “broad, flat;” Av. pərətu- “broad, wide;” Skt. prthu- “broad, wide, large”).

Etymology (PE): Taxt “flat;” Mid.Pers. taxtag “tablet, plank, (chess)board.”

A manifold with a → Riemannian metric that has
→ zero → curvature.

See also: → flat; → manifold.

A galactic → rotation curve in which the → rotation velocity is constant in the outer parts. The flat component is preceded by a rising curve that shows solid body rotation in the very center of the → galaxy. A flat rotation curve implies that the mass is still increasing linearly with radius. See also → dark matter.

See also: → flat; → rotation; → curve.

A Universe where the → geometry is → Euclidean, i.e. parallel lines remain parallel when extended into the distance and the sum of the interior angles of a triangle is 180°. The → space-time in a flat Universe has a null → curvature constant, k = 0. See also → closed Universe, → open Universe.

See also: → flat; → universe.

Exposure of a diffuse and uniform source in order to calibrate the non-uniformity of an imaging detector such as a CCD.

Etymology (EN): → flat; → field.

Etymology (PE): Meydân, → field; taxt, → flat.

The observed fact that the → geometry of the → Universe is very nearly flat, in other words its density is very close to the → critical density. This would be an extreme coincidence because a → flat Universe is a special case. Many attempts have been made to explain the flatness problem, and modern theories now include the idea of → inflation.

See also: → flat; → problem.

11 Taste, especially the distinctive taste of something as it is experienced in the mouth. A particular quality noticeable in a thing (Dictionary.com).

2. Any of the six labels given to the distinct kinds of → quark: up, down, strange, charm, bottom, and top.
   

3. → neutrino flavor.

Etymology (EN): M.E., from O.Fr. flaor “smell, odor; action of smelling, sense of smell,” probably from V.L. flator “odor,” literally “that which blows,” in L. “blower,” from flareQ “to blow, puff,” which is cognate with O.E. blawan, → blow.

Etymology (PE): Câšni “taste; taste by way of a sample; quality,” related to
cašidan, caš- “to taste,” câšt “breakfast,” cašté “bait;”
Mid.Pers. câšt “meal,” câšnig “taste;” cf. Skt. cas- “to eat;” Proto-Ir. caš- “to eat, to drink; to drip.”

Two rules used to assist in remembering the relative directions of the magnetic field, current, and motion in electrical machines, using one’s fingers. The right hand refers to generators, the left hand to motors. The three directions are represented by the thumb (for force or motion), forefinger (for field), and second finger (for current), all held at right angles to each other.

See also: Devised by the British physicist and electrical engineer John Ambrose Fleming (1849-1945).

1. To bend or be bent.
   

   2. Of a material, to be capable of warping or bending and then reverting to shape (OxfordDictionaries.com).

Etymology (EN): Probably a back-formation from → flexible, from L. flexus, p.p. of flectere “to bend, turn.”

Etymology (PE): Caftidan, caftan “to bend, incline,” cafté “curved, bent,” of unknown origin.

1. The quality of bending easily without breaking.
   

   2. The ability to be easily modified.

See also: → flexible; → -ity.

Capable of being bent easily without breaking.

See also: → flex + -ible, variant of → -able.

1. The act of bending, the state of being bent.
   

   2. The position that a limb assumes when it is bent.

See also: → flex; → -tion.

The action of bending or curving, or the condition of being bent or curved.

See also: → flex + suffix -ure.

The act, manner, or power of flying.

Etymology (EN): O.E. flyht “a flying, flight,” from P.Gmc. *flukhtiz (cf. Ger. Flucht).

Etymology (PE): Parvâz, from Mid.Pers. parwâz; Av. pāirivāza- “overrunning,” from pāiri- “over (of space), from” + vāza- from vaz- “to fly, float; to drive;” cf. Skt. vah-, vahati. Also Mid.Pers. vâz “jump, flight,” Mod.Pers. Lori, Gilaki, Tabari vâz “jump, leap.”

A basic type of optical glass containing lead oxide and a smaller amount of potassium, characterized by its brilliance, clarity, and durability. It has a high → dispersion relative to its → refractive index, as compared to, e.g. → crown glass. Flint glass is sometimes used as the diverging lens component of an → achromatic lens. It is also used in the manufacture of table glassware.

See also: Flint, because it was originally made of calcined flints,
from M.E., O.E.; cf. M.Du. vlint, O.H.G. flins, Dan. flint; Gk. plinthos “brick, tile.”

A galaxy that has short segments of patchy spiral structure so that the disk appears like the fleece of a sheep. Examples: NGC 2841 and NGC 5055.

Etymology (EN): From L. floccus “flock of wool” + -ulent; → spiral galaxy.

Etymology (PE): Kahkešân, → galaxy; mârpic, → spiral; pašmin “woolly, woollen,” from pašm “wool” (Mid.Pers. pašm “wool;” Av. pašna- “eyelash, eyelid;” cp. Skt. páksman- “eyelashes;” Gk. pekos “wool, fleece,” pek(t)ein “to comb, pluck;” Lith. pešti “to pluck;” O.N. fax “mane”).

The overflowing of the normal confines of a stream or other body of water, or the accumulation of water over areas that are not normally submerged.

Etymology (EN): O.E. flod “a flowing of water, river, sea,” from P.Gmc. *flothuz (cf. M.Du. vloet, Ger. Flut), from PIE *plo-/*pleu- “flow, float” (cf. Gk. ploein “to float, swim”).

Etymology (PE): Laf “flood,” from Lori, Kordi, Malâyeri, lé in Tabari, variants
lur, lây “flood” [Mo’in, Dehxodâ];
cf. Gk. louein “to wash;” L. luere “to wash;”
Bret. laouer “trough;” PIE *lou- “to wash.” Seyl from Ar.

1. That part of a room, hallway, or the like, that forms its lower enclosing surface and upon which one walks.
   

2. A level, supporting surface in any structure (Dictionary.com).

Etymology (EN): M.E. flor, from O.E. flor (cf. M.Du. vloer, M.H.G. vluor, Ger. Flur “field, meadow”), from PIE *plaros, from *pele- “flat; to spread.”

Etymology (PE): Kaf “floor; plain ground.”

An → asteroid family that includes → Gaspra. The group has about 819 members and orbits between 2.17 and 2.33 → astronomical units from the Sun.

See also: → Flora; → family.

1. The act or state of remaining on the surface of a liquid. → buoyancy.
   

2. The process of separating different materials, especially minerals, by agitating a pulverized mixture of the materials with water, oil, and chemicals based on their tendency to sink in, or float on.

Etymology (EN): From float, from M.E. floten,  O.E. flotian (cf. O.N. flota, M.Du. vloten), akin to fleet + -ation.

Etymology (PE): Šenâvari “flotation,” from šenâvar “that swims, floats,” from šenâ “swimming;” Mid.Pers. šnâz “swim,” šnâzidan “to swim;” Av. snā- “to wash, swim;” cf. Skt. snā- “to bathe, to wash;” L. nare, natare “to swim” (Fr. nage, nager, natation; Sp.nadar, natacion).

1a) Moving along in a → stream; going as in a stream.

1b) A → dynamical system that evolves continuously with time.

1c) The transference of energy.

2. The corresponding verb.

Etymology (EN): O.E. flowan, from P.Gmc. *flo- (cf. Du. vloeien “to flow,” O.H.G. flouwen “to rinse, wash”), probably from PIE *pleu- “to flow, float” (cf. Skt. plavate “navigates, swims,” plavayati “overflows;” Gk. plyno “I wash,” pleo “swim, go by sea;” L. pluere “to rain;” Arm. helum “I pour;”
Lith. pilu “to pour out”).

Etymology (PE): 1) Tacân, from tac- variant tâz- present stem of tacidan, tâxtan, tâzidan “to run; to hasten; to assault,”

• noun and adj. suffix -ân. Related to the first component are
  Mod.Pers. tajan name of a river (initially “flowing, streaming, stream”), tâzi “swift (greyhound),” tak “running, rush,” from
  Mid.Pers. tâz-, tâxtan “to flow, to cause to walk,” tc- “to flow, to walk,” tag “running, attack,” tâzig “swift, fast;”
  Khotanese ttajs- “to flow, to walk;” Av. tac- “to run, to flow,” taciāp- “flowing water,” tacinti (3pl.pers.act.) “to flow,”
  tacar- “course,” tacan “current, streaming;” cf. Skt. tak- “to rush, to hurry,” takti “runs;” O.Ir. tech- “to flow;” Lith. teketi “to walk, to flow;” O.C.S. tešti “to walk, to hurry;” Tokharian B cake “river;” PIE base *tek^w- “to run; to flow.”
  

2. Verbal form.

Same as → streamline.

See also: → flow; → line.

The amount of a substance, specifically a → fluid, moving across a specified unit → area in a given amount of → time.

See also: → flow; → rate.

Same as → stream tube.

See also: → flow; → tube.

The ability of a body of matter (liquid, gas, loose particulate solid) to flow.

Etymology (EN): From flowable, adj. from → flow

• → -able + → -ity.

Etymology (PE): Tacandegi, from tacandé (originally tacandag),
agent noun of tacidan, → flow, + noun suffix -i.

To change continually; to shift back and forth; vary irregularly; to rise and fall in or as if in waves.

Etymology (EN): From L. fluctuationem (nom. fluctuatio), from fluctuare “to undulate,” from fluctus “wave,” from p.p. of fluere “to flow.”

Etymology (PE): Verbal form of oftâxiz, → fluctuation.

Continual rise and fall.

Etymology (EN): Verbal noun of → fluctuate.

Etymology (PE): Oftâxiz “fall and rise,” from oft “fall”
stem of oftâdan “to fall” (Mid.Pers. opastan “to fall,” patet “falls;” Av. pat- " to fly, fall, rush," patarəta- “winged;” cf. Skt. patati “he flies, falls,” pátra- “wing, feather, leaf;” Gk. piptein “to fall,” pterux “wing;” L. penna “feather, wing;” O.E. feðer “feather;” PIE base *pet- “to fly, rush”) + -â- interfix

• xiz present stem of xâstan, xizidan “to rise, get up” (Mid.Pers. xyz- “to stand up, rise;” Proto-Iranian *xiz- “to rise, ascend; increase”).

1. Light, downy particles, as of cotton.
   

2. A soft, light, downy mass (Dictionary.com).

Etymology (EN): Apparently a variant of floow “wooly substance, down, nap,” perhaps from Flemish vluwe, from Fr. velu “shaggy, hairy,” from L. vellus “fleece,” or L. villus “tuft of hair” (Online Etymology Dictionary).

Etymology (PE): Kork “down, soft wool, fluff,” of unknown etymology.

Of, resembling, or covered with fluff.

2. Light or airy (Dictionary.com).

Etymology (EN): From → fluff + -y adj. suffix, from O.E. -ig, cognate with L. -icus, → -ic.

Etymology (PE): Korkvâr “resembling fluff,” with -vâr, a suffix of possession, similarity, and aptitude (e.g., omidvâr, sezâvâr, sugvâr, šâhvâr, gušvâr), → -oid; korki adj. with -i.

An aggregate of small particles loosely stuck together. Same as → porous dust grain.

See also: → fluffy; → dust; → grain.

A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; collective term for liquids and gases.

Etymology (EN): From L. fluidus “fluid, flowing,” from fluere “to flow;” → flux.

Etymology (PE): Šârré, from šârr, → flux.

The branch of → fluid mechanics that deals with the movement of gases and liquids.

See also: → fluid; → dynamics.

The branch of physics that extends the ideas developed in → mechanics and → thermodynamics to the study of motion and equilibrium of → fluids.

See also: → mechanics; → fluid.

The ability of a substance to flow; reciprocal of → viscosity.

See also: → fluid + → -ity.

A type of → luminescence in which photons of lower energy are emitted as the result of absorption of energy by an atom or molecule from other radiation. The phenomenon lasts as long as the stimulus responsible for it is present.

See also: Coined by English mathematician and physicist Sir George G. Stokes (1819-1903) from fluor-, from → fluorspar, + → -escence, a suffix of nouns denoting action or process, change, state or condition, etc.

Possessing the property of → fluorescence.

See also: → fluorescence; → -escent.

A tubular → electric discharge lamp in which light is produced by the → fluorescence → phosphors coating the inside of the tube.

See also: → fluorescent; → lamp.

A compound (salt of hydrofluoric acid) containing → fluorine.

See also: → fluorine; → -id.

Gaseous chemical element; symbol F. It is the most reactive of all chemical elements. It is a yellowish, corrosive gas, which reacts with most organic and inorganic substances. → Atomic number 9; → atomic weight 18.9984; → melting point -219.62 °C; → boiling point -188.14 °C; → density 1.696 grams per liter at STP; → valence -1. Fluorine and its compounds are used in producing uranium (from the → hexafluoride) and more than 100 commercial fluorochemicals, including many high-temperature plastics.

See also: From L. fluere “flow, → flux,” since fluorspar (CaF₂) was used as a flux in metallurgy because of its low melting point. It was discovered in hydrofluoric acid by the Swedish pharmacist and chemist Carl-Wilhelm Scheele in 1771 but it was not isolated until 1886 by the French pharmacist and chemist Ferdinand Moisson.

A → mineral composed of calcium fluoride, CaF₂, also called → fluorspar, that is often → fluorescent under → ultraviolet light.

See also: From fluor-, → fluorine, + → -ite.

Same as → fluorite.

See also: From fluor-, → fluorine, + spar “a crystalline mineral.”

The amount of energy, fluid, or particles passing in a given direction in a unit of time.

Etymology (EN): O.Fr. flux, from L. fluxus, p.p. of fluere “to flow,” PIE base *bhleug- (cf. L. flumen “river;” Gk. phluein “to boil over, bubble up,” phlein “to abound”).

Etymology (PE): Šâr “outpouring of water, wine, etc.,” šâridan “to flow (as rivers), with a great noise; to pour,” âbšâr “waterfall;” saršâr “overflowing; brim-full;” Ossetic ægzælyn “to pour down;” Pashto zγâstəl/zγâl- “to swim;” Av. γžār- “to flow;” cf. Skt. ksar- “to flow;” Gk. phtheirein “to destroy, perish.”

The → calibration of the flux received by a detector in terms of absolute units.

See also: → flux; → calibration.

Flux of radiation that falls on a detector per unit surface area of the detector per unit bandwidth of the radiation per unit time.

See also: → flux; → density.

In radio astronomy, same as → jansky (symbol Jy), a unit of electromagnetic flux equivalent to 10^-26 watts per square meter per Hertz.

See also: → flux; → density.

In Newton’s work on → calculus, the rate of change of a fluent (i.e. a flowing quantity), today commonly known as → variable. For a fluent x, the fluxion is denoted dx/dt. An obsolete mathematical term.

See also: From L. → fluxion-, stem of fluere “to flow,” → flux.

To move through the air using wings. Travel through the air or outer space.

Etymology (EN): M.E. flien, O.E. fleogan; cognate with O.H.G. fliogan, Ger. fliegen, O.Norse fljuga.

Etymology (PE): Infinitive from parvâz, → flight.

A trajectory that takes a → space probe close to a Solar System body (planet, satellite, asteroid, comet) but does not permit it to enter an orbit about the body.

Etymology (EN): → fly + by, from M.E., from O.E. be or bi, from P.Gmc. *bi “around, about” (cf. Du. bij, Ger. bei “by, at, near”), from *umbi, (cf. L. ambi “around, about,” Gk. amphi “around, about,” Skt. abhi “on both sides,”
Av. aibi, aiwi, O.Pers. aiby “to, against, in addition to,” Mid.Pers. aw-, ab-, Mod.Pers. af- (as in afzudan “to increase, add,” afruxtan “to inflame, kindle, blaze,” afqân “lamentation, groaning, cires for help”).

Etymology (PE): Barvâz, from bar- “on, up; upon, over,” → on-,

• vâz, as in parvâz, → flight.

Of or relating to a focus; placed at or measured from a focus.

See also: → focus + → -al.

same as → focal length.

See also: → focal; → distance.

The distance between the optical center of a lens, or the surface of a mirror, and its focus.

See also: → focal; → distance.

A plane at right angles to the principal axis of a lens or mirror on which the best image is formed.

See also: → focal; → plane.

Same as → focus.

See also: → focal; → point.

The ratio of the → focal length of a reflecting surface or lens to its effective diameter, i.e. to its → aperture. The smaller the focal ratio, the smaller the image scale and the more luminous the image for a given aperture.

See also: → focal; → ratio.

An optical component or system for changing the image scale of a telescope to achieve a better match between the → seeing disk and the → pixel size.

See also: → focal; → reducer.

1. (n.) A point where parallel light rays from an object are gathered together by a lens or a concave mirror. It is the place where the clearest image of a distant object forms. Also called focal point. See also → focal distance.
   

2. (v.) To adjust the eyepiece or objective of a telescope so that the image is clearly seen by the observer.

Etymology (EN): From L. focus “hearth, fireplace,” of unknown origin,

Etymology (PE): Kânun “hearth, fireplace.”

Of an optical system, being in focus or brought into focus; adjusted to produce a clear image.

See also: Past participle of → focus.