An Etymological Dictionary of Astronomy and Astrophysics

1. Logic: Relating to or involving inductive reasoning from particular facts or effects to a general principle (dictionary.com).
   

2. Logic: Derived from or requiring evidence for its validation or support; empirical; open to revision (dictionary.com). → a priori.

Etymology (EN): L., literally “from what comes after,” from posterior “after, later, behind,” comparative of posterus “coming after, subsequent,” from post “after,” → post-.

Etymology (PE): Afdom “the last,” Dehxodâ: afdom “last, end;” Mid.Pers. abdom, apdom; from Proto-Ir. *apatama- superlative of *apa- “away from, from;” cf. Av. apəma- “the last, the most distant;” Skt. apamá-.

1. Logic: Relating to or involving deductive reasoning from a general principle to the expected facts or effects (dictionary.com).
   

2. Logic: Known to be true independently of or in advance of experience of the subject matter; requiring no evidence for its validation or support (dictionary.com). → a posteriori.

Etymology (EN): L., literally “from what comes first,” from priori, ablative of prior
“former, previous, first,” → per-.

Etymology (PE): Partom “the first,” Dehxodâ: fardom; Mid.Pers. fradom, fratom, pratom; O.Pers. fratama-; Av. fratəma- (Skt. prathama- “earliest, initial, first”) superlative of fra- “before, forward, forth,” cognate with Gk. and L. → pro-.

One of → Saturn’s rings, lying beyond the → B ring at 122,170 km from the center of Saturn, with a width of 14,600 km.

See also: → ring.

A star whose spectrum shows strong hydrogen → Balmer lines accompanied by many faint to moderately strong metallic lines. In contrast to B and O stars, the lines of
He I and He II are absent. Their surface temperature ranges from 7,500 to 10,000 K. The main metallic lines seen in A-type stars are those of
Fe I and Fe II, Cr I and II, Ti I and II, and account for about two-thirds of all lines. Some famous examples of A star are: → Sirius; → Deneb, → Altair, and → Vega. Also known as A-type star.

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

Prefix meaning “not.”

Etymology (EN): From Gk. a-, an- “not,” compare with O.Pers./Av. a-, an- “not, without,” Skt. a-, an- “not,” L. in-, a variant of PIE *ne- “not”.

Etymology (PE): A-, an-, from O.Pers./Av. negation prefix appearing before consonants and vowels respectively.
A couple of examples in Mod.Pers.: amordâd “immortality, name of the fifth month in the Iranian calendar,” anušé; “fortunate, happy,” anirani “non Iranian,” âhu “vice, defect,” âsoqdé “unburnt, half-burnt wood.”
Bi- privative prefix, from Mid.Pers. abi-, O.Pers. *apaiy-, Av. apa-.
Nâ- negation prefix “not, no,” Mid.Pers. ne, O.Pers. naiy, Av. na-, (particle of negation noit), Skt. na-, (ned), Gk. né- “not,” L. ne-, in-, un-, PIE *ne-.

Same as → A star.

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

→ 1I/‘Oumuamua.

See also: → asteroid designation.

A → thought experiment conceived by Einstein (1879-1955) at the age of sixteen in the Swiss town of Aarau where he attended the Argovian cantonal school. If an → observer moved at the → speed of light, pursuing a → beam of light, would he → observe such a beam of light as a spatially oscillatory → electromagnetic field at rest? The answer came some ten years later from Einstein himself by his theory of → special relativity. Accordingly, the speed of light is constant for all observers and no observer can move at the light velocity.

See also: Aarau, the Swiss town, the capital of the northern Swiss canton of Aargau; → paradox.

A → quadruple system of stars consisting of two close pairs, AB Dor A / AB Dor C and AB Dor Ba / AB Dor Bb, separated by about 9 arcseconds. The brightest star of the system, AB Dor A, is a → pre-main sequence star of → spectral type K1, with strong emission at all wavelengths, from radio to X-rays. The companion of this star, i.e. component C, lying 0.185 arcseconds apart, is a very low-mass star of 0.090
→ solar masses with a spectral type M 5.5 (Boccaletti et al. 2008, A&A 482, 939 and references therein).

See also: A, a, B, b, and C designating letters of alphabet; → Dorado.

“From the beginning,” as opposed to in medias res “starting in the middle of the story.” In physics and chemistry “from first principles;” → ab initio calculation.

Etymology (EN): L. “from the beginning,” from ab “from,” → ab- + initio, ablative of initium “a beginning, an entrance,” from inire “to go into, begin,” from in- “into, in” + ire “to go” (cf. Gk. ienai “to go,” Skt. eti “goes,” O.Pers. aitiy “goes,” Av. aeiti “goes,” Mod.Pers. âyad “comes,” PIE *ei- “to go, to walk”).

Etymology (PE): Hac bon, from Mid.Pers. hac “from” (O.Pers. hacā “from,” Av. hacā “from, out of,” cf. Skt. sácā “with,” PIE base *sek^w- “to follow,” Av. hac-, hax- “to follow,” hacaiti “follows,” Skt. sácate “accompanies, follows,” Gk. hepesthai “to follow,” L. sequi “to follow,” Lith. seku “to follow”)

• bon “beginning, base, root” (Av. būna- “base, depth,” cf. Skt. bundha-, budhná- “base, bottom,” Pali bunda- “root of tree”).

In physics and chemistry, a calculation that relies on basic and established laws without additional assumptions or special models. Experimental input in ab initio calculations is limited to the determination of values of fundamental physical constants.

See also: → ab initio; → calculation.

A → photometric system defined by reference to → monochromatic magnitudes in such a way that, when monochromatic → flux f_ν is measured in ergs sec^-1 cm^-2 Hz^-1, the magnitude will be:

AB = -2.5 logf_ν - 48.60.

The constant is set so that AB is equal to the V magnitude for a source with a flat → spectral energy distribution. The → zero point is defined by the flux of the star → Vega at 5546 Å. In this system, an object with constant flux per unit frequency interval has zero color.

See also: → magnitude; → system.

From, away, off.

Etymology (EN): M.E., from O.Fr., from L. ab “from, away from, out of, with” Gk.

See also: → apo-.

1. To leave completely and finally; forsake utterly; desert.
   

2. To give up; discontinue; withdraw from (Dictionary.com).

Etymology (EN): M.E. abando(u)nen, from M.Fr. abandoner, from O.Fr. abandoner from adverbial phrase à bandon “at will, at discretion,” from à “at, to,” → ad-, + bandon “power, jurisdiction,” from L. bannum “proclamation.”

Etymology (PE): Râcidan, related to Pers. parhêz, parhiz “to keep away from, abstain, avoid,” gurêz, goriz “to flee, run away;” Av. raēc- “to leave, let;” → heritage.

The reciprocal of the → dispersive power of a substance. Also known as constringence.

See also: → Abbe sine condition; → number.

In → geometric optics, a condition for eliminating → spherical aberration and → coma in an → optical system. It is expressed by the relationship: sin u’/U’ = sin u/U, where u and U are the angles, relative to the → optical axis,
of any two rays as they leave the object, and u’ and U’ are the angles of the same rays where they reach the image plane. A system which satisfies the sine condition is called → aplanatic.

See also: Named after Ernst Karl Abbe (1840-1905), a German physicist; → sine; → condition.

The → locus of the intersection of input and output → conjugate rays in an
optical system satisfying the → Abbe sine condition.

See also: → Abbe sine condition; → sphere.

1. If a → power series → converges for some nonzero value x₀, then it converges absolutely for any value of x, for which |x| < |x₀|.
   

2. If a power series → diverges for some nonzero value x₀, then it diverges for any value of x, for which |x| > |x₀|.

See also: Named after the Norwegian mathematician Niels Henrik Abel (1802-1829); → theorem.

A catalog of 4073 rich → galaxy clusters grouped by constellation and by catalog number. Published first in 1958, it contained the clusters visible from the northern hemisphere. A supplement for the southern hemisphere survey was published in 1988.

See also: George O. Abell (1927-1983), American astronomer; → catalog.

Diverge or deviate from the straight path; produce → aberration.

Etymology (EN): Aberrate, from aberrare “go astray,” from ab- “away” + errare “to wander.”

Etymology (PE): Birâhidan, from birâh “a devious path; a wanderer, who deviates, errs,” from bi- “without” + râh “way”.

1. An imperfection in the imaging properties of a → lens or → mirror.
   The main aberrations are → chromatic aberration, → spherical aberration, → coma, → astigmatism, → distortion, and → field curvature.
   

2. → aberration of starlight.
   

See also: → aberration angle→ aberration constan→ aberration of starlight→ aberration orbit→ annual aberration→ chromatic aberration→ constant of aberration→ diurnal aberration→ optical aberration→ planetary aberration→ relativistic aberration→ secular aberration→ spherical aberration→ stellar aberration.

Etymology (EN): Aberration, from L. aberrationem, from aberrare “go astray,” → aberrate.

Etymology (PE): Birâheš, from birâidan, → aberrate.

The angle tilt required by the → stellar aberration phenomenon in order that a moving telescope points directly to a star.

See also: → aberration; → angle.

Same as → constant of aberration.

See also: → aberration; → constant.

→ aberration of starlight

See also: → aberration; → light.

An apparent displacement in the observed position of a star. It is a result of the finite speed of light combined with the relative motion of the Earth through space. Suppose that you walk through a vertically falling rain with an umbrella over your head. The faster you walk, the further you must lower the umbrella in front of yourself to prevent the rain from striking your face. For starlight to enter a telescope, a similar phenomenon must occur, because the Earth is in motion. The telescope must be tilted in the direction of motion by an angle: tan θ =(v/c), where v the Earth velocity and c the speed of light. The aberration of starlight was discovered by the English astronomer James Bradley (1693-1762) in 1729 by observing → Gamma Draconis. The tilt angle is θ = 20’’.50, from which the Earth’s orbital speed, 29.80 km s^-1, can be deduced, using the above equation. See also → annual aberration; → diurnal aberration; → secular aberration. → Special relativity modifies the classical formula for aberration, predicting results which differ substantially from those of classical physics for objects moving at a substantial fraction of the speed of light; → relativistic aberration.

See also: → aberration; → star;
→ light.

The apparent path described by a star on the → celestial sphere due → annual aberration. A star at the → ecliptic pole is seen to move around a circle of angular radius about 20".50, once a year. A star on the → ecliptic oscillates to and fro along a line of angular half-length 20".50. At an intermediate → celestial latitude, β, the aberration orbit is an ellipse, with semi-major axis 20".50 and semi-minor axis (20".50) sin β.

See also: → aberration; → orbit.

Of or pertaining to → aberration.

See also: → aberration; → -al.

A → Besselian day number denoted by C or D.

See also: → aberration; → -al; → day; → number.

The → locus of points on the → celestial sphere occupied by a star during the annual → revolution of the → Earth about the → Sun due to → annual aberration. annual aberration.

See also: → aberrational; → ellipse.

The → erosion of a surface through a process such as → vaporization or → friction.

Etymology (EN): L. ablatio, ablation, from ablatus, from ab- “away” + latus “carried.”

Etymology (PE): Farsâb from far-, prefix denoting “abundance, excess” + sâb present stem of sâbidan “to rub, wear out,” variants sâyidan,
pasâvidan “to touch,” Khotanese sauy- “to rub,” Sogdian ps’w- “to touch,” Proto-Iranian *sau- “to rub.”

Not typical, usual, or regular; not normal; deviant. → anomalous, → anomaly.

Etymology (EN): M.L. anormalis, blend of L.L. abnormis “deviating from a rule,” from ab- “off, away from” + L. norma “rule.”

Etymology (PE): Bihanjâr, from Pers. bi- “without” +
Pers. hanjâr “rule, habit, law, staright road.”

1. To terminate a procedure before it has finished naturally.
   

2. In computer science, to terminate an application intentionally or unexpectedly.
   For example, to stop a search or a print job; or unexpected
   termination of a program because of bugs or difficulties.

Etymology (EN): From L. abortus, p.p. of aboriri “to miscarry” (a child) from → ab- “from, away, off,” + oriri “to rise, be born;” cf. Pers. rasidan
“to attain; to arrive” (Mid.Pers. rasidan “to arrive, to mature;” O.Pers./Av. rasa- present stem of ar- “to move, go or come toward;” Skt. ar-, rcchati “reaches;”
Gk. erkhomai “to go, to reach”).

Etymology (PE): Fagânidan, from fagâné “abortion”, related to fegandan, fekandan, afkandan “to throw, to cast away;” Mid.Pers. abgandan “to throw;” O.Pers. avakan- “to throw, place on,” from Proto-Iranian *kan- “to throw, place, put.”

The stopping of a process; a result of such termination.

See also: Verbal noun of → abort.

In plane Cartesian coordinates, the distance of any point from the vertical axis (y-axis). The distance from the horizontal axis (x-axis) is called → ordinate.

Etymology (EN): From L abscissa (linea) “(a line) cut off,” from p.p. of abscindere “to cut off,” from → ab- “off, away” + scindere “to cut.”
The word abscissa was first used by Stefano degli Angeli (1623-1697), a professor of mathematics in Rome.

Etymology (PE): Pirâ, present stem of pirâstan “to prune, clip, trim; to adorn, embellish (especially by cutting, clipping, or taking away),” related to ârâstan “to arrange, adorn,” from Mid.Pers. payrâstan, patrâstan “to arrange, adorn,” ultimately from Proto-Iranian *pati-rad-. The first component *pati- “to, toward, near to, against;” cf. Mid.Pers. pât-, from O.Pers. paity “against, back, opposite to, toward, face to face, in front of;” Av. paiti;
Skt. práti “toward, against, again, back, in return, opposite;” Pali pati-; Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti. The second component *rad- “to direct, to prepare;” cf. O.Pers. rād- “to prepare,” rās- “to be right, straight, true,” rāsta- “straight, true” (Mod.Pers. râst “straight, true”); Av. rāz- “to direct, put in line, set,” razan- “order;” Gk. oregein “to stretch out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” Skt. rji- “to make straight or right, arrange, decorate;” PIE base *reg- “move in a straight line.”

1. State of being away or not being present.
   

2. Lack; deficiency.

See also: Noun from → absent.

1. Not in a certain place at a given time; away, missing (opposed to present).
   

2. Lacking; nonexistent (Dictionary.com).

Etymology (EN): M.E., from M.Fr. absent, from L. absentem (nominative absens), p.p. of abesse “to be away from,” from → ab- “away” + esse “to be,” → present.

Etymology (PE): Apâst, on the model of L. absentem, as above, from apâ- “away from,” → apo-, + ast “is,” → present.

A person who is absent, especially from work or school (Dictionary.com).

Etymology (EN): From → absent + -ee a suffix forming nouns that denote a person who is the object or beneficiary of the act specified by the verb, from Fr. -é, ending of p.p.s used as nouns.

Etymology (PE): Apâstgar, from apâst, → absent, + -gar, → -or.

Voluntary non attendance at work, without valid reason. Absenteeism means either habitual evasion of work, or willful absence as in a strike action. It does not include involuntary or occasional absence due to valid causes, or reasons beyond one’s control, such as accidents or sickness (BusinessDictionary.com).

See also: → absent; → -ism.

1. General: Not limited by restrictions or exceptions; perfect in quality or nature; unqualified in extent or degree; complete.
   

2. Physics: Relating to measurements or units of measurement derived from fundamental units of length, mass, and time.
   

3. Physics: Having an ideal value as opposed to a conditional or relative one.
   

See also: → absolute acceleration, → absolute dating, → absolute humidity, → absolute luminosity, → absolute magnitude, → absolute permeability, → absolute space, → absolute temperature, → absolute tensor, → absolute time, → absolute viscosity, → absolute zero.

Etymology (EN): From M.Fr. absolut, from L. absolutus “unrestricted,” p.p. of absolvere “to set free,” from ab- “away” + solvere “to loosen,” from PIE *leu-. → solve.

Etymology (PE): Avast from negation prefix → a- + vast, variant of bast, basté “tied, bound,” from Mid.Pers. bastan/vastan “to bind, shut,” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie,” Skt. bandh- “to bind, tie, fasten,” PIE *bhendh- “to bind,” cf. Ger. binden, E. bind, → band.

Synonyms: Synonymous: motalq(مطلق), from Ar.

For a body that moves with respect to a rotating → reference frame, the vector sum of the observed acceleration, the → Coriolis acceleration, and the
→ centrifugal acceleration. See also the
→ Coriolis theorem.

See also: → absolute; → acceleration.

Any method of measuring the age of an event or object in years. For example, in geology, this method can, unlike → relative dating, give us the age of a rock or fossil in x number of years. The most widely used and accepted method of absolute dating is → radioactive dating. See also: → radiocarbon dating, → radiometric dating.

See also: → absolute; → dating.

The difference between the measured value of a quantity x₀ and its (true) actual value x, given by Δx = x₀ - x. See also: → relative error.

See also: → absolute; → error.

In a system of moist air, the ratio of the mass of water vapor to the total volume of the system. → humidity.

See also: → absolute; → humidity.

A star’s → intrinsic brightness, i.e. the total amount of energy radiated by the star per second. → Luminosity is often expressed in units of watts or erg/sec. The Sun’s absolute luminosity is 3.86 × 10³³ erg/sec.

See also: → absolute; → luminosity.

1. The → magnitude a star would have if it were at a distance of 10 → parsecs in a void space, without → interstellar absorption. The absolute magnitude is usually deduced from the → visual magnitude, measured through a V filter (→ UBV system), when it is written as M_V. If it is defined for another wavelength, it gets another index (U, B, etc). If the radiation on all wavelengths is included, it becomes absolute → bolometric magnitude, M_bol. The Sun has the absolute magnitude + 4.8. Most of the stars have absolute magnitudes ranging between -9 (→ supergiants) and + 19 (→ red dwarfs) (M.S.: SDE).
   

2. The brightness a → comet or → asteroid would have if it were at a distance of 1 → astronomical unit both from the Sun and the Earth and were completely illuminated by the Sun (M.S.: SDE).

See also: → absolute; → magnitude.

A measurement in which the comparison is directly with quantities whose units are basic units of the system. For example, the measurement of speed by measurements of distance and time is an absolute measurement, but the measurement of speed by a speedometer is not an absolute measurement. Note that the word absolute measurement implies nothing about → precision or → accuracy (IEEE Standard Dictionary of Electrical and Electronics Terms).

See also: → absolute; → measurement.

→ magnetic permeability.

See also: → absolute; → permeability.

A fixed space in which physical phenomena occur and whose properties do not depend on what occupies it, nor on the observer. It is a distinguished frame of reference that could show bodies to be truly moving or truly at rest. Absolute space is one of the basic assumptions of → Newtonian mechanics, but it was abandoned in Einstein’s → special relativity. See also → absolute time; → space-time.

See also: → absolute; → space.

Also called → thermodynamic temperature, the value of a → temperature in the → Kelvin scale. It is is equal to the temperature on the → Celsius scale -273.15 °C.

See also: → absolute; → temperature.

A → tensor of → weight  → zero.

See also: → absolute; → tensor.

A universal time supposed to be the same for all observers at any place in the Universe. Absolute time is one of the foundations of → Newtonian mechanics, but it fails to account for physical phenomena in → reference frames
with relative motion. Its abandoning was one of the starting points of → special relativity. See also → absolute space; → space-time.

See also: → absolute; → time.

For any → real number a, the non-negative value of a without regard to its sign; denoted by |a|. Same as → modulus. The absolute value of a is always either → positive or → zero, but never negative. The absolute value of a number may be thought of as its → distance from zero. The following rules hold:
|ab| = |a||b|
|a + b|  ≤  |a| + |b|
|a - b|  ≥  |a| - |b|
For a> 0, |x|  ≤  a if and only if -a  ≤  x  ≤  a.

See also: → absolute; → value.

Same as → viscosity and → dynamic viscosity.

See also: → absolute; → viscosity.

The → zero point of the → Kelvin scale of → temperature; the theoretical point at which all molecular activity ceases, -273.16 °C.

See also: → absolute; → zero.

1. General: To suck up or take up.
   

2. Physics: To retain → energy (radiation or sound, for example) or → matter wholly, without → reflection or → transmission.

Etymology (EN): M.Fr. absorber, from L. absorbere “to swallow up,” from ab- “from” + sorbere “suck in;” PIE root *srebh- “to suck, absorb;” cf. Arm. arbi “I drank;” Gk. rhophein “to sup up, gulp down;” Lith. srebiu “to drink greedily;” Proto-Iranian *hrab- “to sip, suck (in),” cf. Pashto raw-, Shaghni râv-, Sariqoli rov-, Ishkashimi ruv- “to suck” (Cheung 2007), Pers. (prefixed) zâlu “leech.”

Etymology (PE): Daršamidan, from daršam, from dar- “in, into”

• šam, variant of šâm, as in âšâm, âšâmidan “to drink, to sip;” Av. šam- “to drink, sip, swallow;” Skt. cam, camati “to sip, drink, lick up, absorb.”

Capable of being absorbed.

Etymology (EN): Absorbable, adj. from → absorb + → -able.

Etymology (PE): Daršamidani adj. from aršamidan “to absorb”

• -i, → -able.

The ability of a solution or a layer of a substance to absorb → radiation, i.e. the ratio of the intensity of the light incident on the solution to the intensity transmitted by it, in logarithmic scale.

See also: → absorb; → -ance.

1. Any material or → substance that → absorbs.
   

2. Relating to or capable of → absorption.

Etymology (EN): Absorbent, from → absorb + -ent.

Etymology (PE): Daršamandé from aršam present stem of daršamidan, → absorb, + -andé suffix forming adj. and/or agent name.

Any medium that absorbs something.

See also: → absorb; → -er.

Optics: A strip or annulus of material, such as glass or film, whose density increases progressively from one end to the other. The function of the wedge is to weaken the light beam in an optical system.

See also: Verbal adj. from → absorb; → wedge.

1. General: The process or fact of absorbing.
   

2. Physics: The action of energy or matter penetrating or being assimilated into a body of matter with no reflection or emission. → adsorption,
   → desorption, → sorption.
   

See also: → absorption band, → absorption coefficient, → absorption curve, → absorption feature, → absorption line, → absorption nebula, → absorption spectrum, → atmospheric absorption, → discrete absorption component, → foreground absorption, → intergalactic absorption, → internal absorption, → interstellar absorption, → mass absorption coefficient, → photoabsorption, → selective absorption, → self-absorption.

See also: Verbal noun of → absorb; → -tion, from L. absorptionem.

1. A series of very closely spaced absorption lines in stellar spectra resulting from the absorption of light by molecules. Bands caused by titanium oxide (→ TiO bands) and carbon compounds occur in the spectra of low temperature M and C stars.
   

2. A range of wavelengths, usually in electromagnetic radiation, that are absorbed by a given substance. Absorption bands are characteristic of molecules and correspond to changes of electron orbits in the molecules. See also → anomalous dispersion.

See also: → absorption; → band.

The fraction of normally incident light that is absorbed per unit path length or by a unit mass of absorbing medium.

See also: → absorption; → coefficient.

A graphic representation of the amount of radiant energy absorbed by a material as a function of the wavelength.

See also: → absorption; → curve.

A dip in a spectrum indicating that light has been absorbed on the path from the emitting source to the observer. It may be a line, corresponding to a well-defined atomic or molecular transition, or a more complex feature whose physical origin is not necessarily understood.

See also: → absorption; → feature.

A dark line in the spectrum of a source produced by a lower temperature gas cloud lying between the source and the observer.

See also: → absorption; → line.

A dark cloud of dust and gas that absorbs light from and impedes the view of background stars; dark nebula.

See also: → absorption; → nebula.

A spectrum of absorption lines or bands, produced when light from a hot source, itself producing a continuous spectrum, passes through a cooler gas.

See also: → absorption; → spectrum.

The ratio of energy absorbed by a body to energy incident upon the same body.

Etymology (EN): Absorptivity, from absorptive adj. from → absorb + → -ity, suffix expressing state or condition.

Etymology (PE): Daršamâyi, from daršamâ quality adj. from daršam, present stem of daršamidan + -yi noun suffix.

For a spatial object, the ratio between its absorption of solar radiation and its infrared emission.

See also: → absorptivity; → emissivity; → ratio.

To keep oneself from doing or partaking of something; refrain. → avoid.

Etymology (EN): M.E. absteynen, absteinen, abstenen, from O.Fr. abstiner, abstenir, astenir “hold (oneself) back, refrain voluntarily,” from L. abstinere / abstenere “withhold, keep back,” from → ab-“off, away from” + tenere “to hold,” from PIE root *ten- “to stretch,” → tension.

Etymology (PE): Parhiz, from Mid.Pers. pahrêzidan, pahrêxtan “to keep away from, abstain,” ultimately from Proto-Ir. *pati-raic-, from prefix *pati-

• *raic- “to leave, let abandon;” cf. Av. raēc- “to leave, let;” Pers. (+ *ui-) gorêz-, goriz-, gorixtan “to run away, flee;” Skt. rec- “to leave, release;” Gk. leipo “I let;” L. linquô “I leave;” PIE base *leik^w- “to leave, let.”

The act or practice of abstaining from indulging a desire or appetite.

See also: Verbal noun of → abstain.

1a) (adj.) A designation of what derives from → abstraction. Contrasted to → concrete.

1b) Dissociated from any concrete reality or specific instance.

2a) (n.) An abstract thing or state.

2b) A summary of a scientific article, document, speech, etc.

Etymology (EN): M.E., from L. abstractus “drawn away,” p.p. of abstrahere “to draw away, withdraw,” from abs- “away,” from → ab-

• trahere “to draw,” → attract.

Etymology (PE): 1) Âhanjidé, p.p. of âhanjidan, variants âhixtan, âxtan “to draw, pull, extract,” → object.

2. Cekidé, literally “dropped, oozed out,” from cekidan “to drop, distill, fall in small portions, as water,” from cek, cekké “a drop, as water,” probably an onomatopoeia.

An entity that does not exist in space or time and is not perceptible. Some examples are universals, sets, geometrical figures, and numbers.

See also: → abstract; → object.

1. The mental process in which an element or quality is separated from a total object. Also the result of this process.
   

2. Chemistry: → abstraction reaction.

See also: Verbal noun from → abstract; → -tion.

Chemistry: A bimolecular chemical reaction that involves removal of an atom or ion from a molecule. For example, hydrogen abstraction from methane:
CH₄ + Cl → CH₃ + HCl.

See also: → abstraction; → reaction.

Ridiculously senseless, illogical, or untrue; contrary to all reason. → reductio ad absurdum, → absurdity. See also → nonsense.

Etymology (EN): M.Fr. absurde, from L. absurdus “out of tune; foolish,” from surdus “deaf, stupid, inaudible.”

Etymology (PE): Yâvé, literally “unrelated speech,” ultimately from *ui-vaka-, from *ui- “apart, away,” → transit, + *vaka- “word, speech,” → word.

The quality or state of being absurd; something that is absurd.

See also: → absurd + → -ity.

1. The relative amount of a given → chemical element with respect to other elements.
   

2. The amount of an → isotope relative to other isotopes of the same element in a given sample. → overabundance; → underabundance.

Etymology (EN): M.E., from O.Fr. abundance, from L. abundantia “fullness,” from abundare “to overflow,” from L. → ab- “away” + undare “to surge,” from unda “water, wave.”

Etymology (PE): Farâvâni “abundance,” from farâvân “abundant,” from feré “much, more; increase; priority;”
Mid.Pers. frêh “more, much;” O.Pers. fra- “before, forth;”
Av. frā, fərrā “forth, forward;” PIE base *pro-; cf. L. pro “on behalf of, in place of, before, for;” Gk. pro- “before, in front of.”

1. Of or relating to a college, academy, school, or other educational institution, especially one for higher education.
   

2. Pertaining to areas of study that are not primarily vocational or applied, as the humanities or pure mathematics.
   

3. Theoretical or hypothetical; not practical, realistic, or directly useful (Dictionary.com).

See also: → academy; → -ic.

A member of an association or institution for the advancement of arts, sciences, or letters.

Etymology (EN): From Fr. académicien, from académic-, → academic,

• -ian a suffix forming adjectives and nouns.

Etymology (PE): Farhangestânvand, from farhangestân, → academy,

• -vand a suffix forming adjectives and agent nouns, → actual.

1. A group of authorities and leaders in a field of scholarship, art, etc., who are often permitted to dictate standards, prescribe methods, and criticize new ideas.
   

2. An association or institution for the advancement of art, literature, or science (Dictionary.com).

Etymology (EN): From Fr. Académie, from L. Academia, from Gk. Akademeia “grove of Akademos,” a legendary Athenian of the Trojan War tales, whose estate, six stadia from Athens, was the enclosure where Plato taught his school.

Etymology (PE): Farhangestân, literally “site of culture and education,” from farhang, → culture, + -estân suffix of place and time, → summer.

(v.tr.) To increase the velocity of a body; to cause to undergo acceleration.
(v.intr.) To increase in speed.

See also: Verbal form of → acceleration.

The motion of an object subject to → acceleration. Opposite to → uniform motion.

Etymology (EN): Accelerated, from → accelerate; → motion.

Etymology (PE): Jonbeš, → motion; šetâbdâr “accelerated,” from šetâb→ accelerate + dâr “having, possessor” (from dâštan “to have, to possess,” Mid.Pers. dâštan, O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind;” cf.
Skt. dhr-, dharma- “law;” Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support”).

Moving or proceeding with → acceleration; e.g.
→ accelerating system; → accelerating Universe.

See also: Verbal adj. from → accelerate.

→ accelerating Universe.

See also: → accelerating; → expansion; → Universe.

A material system that is subject to a constant force in each and every one of its instantaneous points of trajectory.

See also: → accelerating; → system.

The deduction based on the observation that the most distant → Type Ia supernovae
are fainter than that expected from their → redshifts in a matter-only dominated expanding Universe. The faintness is attributed to larger distances resulting from an accelerating Universe driven by presence of a new component with strongly negative pressure. This
component that makes the Universe accelerate is named → dark energy. The deceleration or acceleration of an expanding Universe, given by the general relativistic equation, is:
R^../R = -(4/3)πGρ(1 + 3w),

where R is the linear → cosmic scale factor of the expanding Universe, G the → gravitational constant, ρ the mean density of the Universe, and w the → equation of state parameter representing dark energy. The expansion accelerates whenever w is more negative than -1/3. The Nobel Prize in Physics 2011 was awarded to the initiators of this concept,
Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess, for their discovery of the accelerating expansion of the Universe through observations of distant supernovae. See also the original paper: Perlmutter et al. 1999, ApJ 517, 565.

See also: → accelerating; → universe.

The rate at which the velocity of an object changes with time.

Etymology (EN): Acceleration, from accelerate, from L. accelerare “quicken,” from → ad- “to” + celerare “hasten,” from celer “swift” (cf. Skt. car, carati “to move, go, drive,” Gk. keles “fast horse, horse race,” Av. kar- “to walk, move, go around,” Mod.Pers. cal, calidan “to move, to go, to walk” (jald? “quick, active, brisk”), Gilaki/Hamadâni jal “quick, fast,“Lori žil “motion, impulse”); PIE *kel- “to drive, set in swift motion.”

Etymology (PE): Šetâb “quickness, haste, speed,” Mid.Pers. ôštâp “hurry, haste,” ôštâftan “to hurry, hasten,” from *abi.stap-, from
abi- " to; in addition to; against”

• *stap- “to oppress,” Arm. (loanword) štap “haste, trouble.”

The acceleration that an object experiences because of gravity when it falls freely close to the surface of a massive body, such as a planet. Same as → gravitational acceleration.

See also: → acceleration; → gravity.

A measure of the departure from a constant rate of the acceleration of the Universe, expressed by:
q(t) = R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. Traditionally, a negative sign is inserted in the above equation for the → deceleration parameter.

See also: → acceleration; → parameter.

A machine which accelerates charged atomic (ions of various elements) or subatomic particles (usually electrons and protons) to high energies. → cyclotron; → synchrotron.

Etymology (EN): Accelerator, from accelerate, → acceleration,

• -or agent suffix, from M.E. -or, -our, from O.F. -eor, -eur, from L. -or.

Etymology (PE): Šetâbgar, from šetâb→ acceleration + -gar agent suffix, → -or.

In a → semiconductor, an → impurity which creates a hole in the electronic structure of the crystal and causes p conduction. → donor; → impurity.

Etymology (EN): M.E. acceptour, from O.Fr. acceptour, from L. acceptor, from acceptare “take or receive willingly,” from accipere “to receive,” from → ad- “to” + capere “to take, hold, seize,” PIE base *kap- “to grasp” (cf. Skt. kapati “two handfuls,” Gk. kaptein “to swallow,” Goth. haban “have, hold,” O.E. habban “to have, hold,” probably Mod.Pers. qâp-, qâpidan, kapidan “to seize, rob,” Av. haf-, hap- “to keep, observe”).

Etymology (PE): Pazirandé agent noun from paziroftan “to receive, accept, admit,” Mid.Pers. padiriftan, padir- “to receive, accept,” from Proto-Iranian *pati- + *grab- “to grab, seize, take,” Av./O.Pers. grab- “to take, seize;” cp. Skt. grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving,” M.L.G. grabben “to grab,” from P.Gmc. *grab (E. grab); PIE *ghrebh- “to seize.”

1. To have the right to connect to a computer or a server and retrieve information.
   
2. The act or instance of accessing a computer.

Etymology (EN): M.E. accesse, from O.Fr. acces “onslaught,” from L. accessus “a coming to, an approach,” p.p. of accedere “to approach, enter upon” from → ad- “to” + cedere “go, move.”

Etymology (PE): Dastrasi “access,” from dast “hand” (Mid.Pers. dast; O.Pers. dasta-; Av. zasta-; cf. Skt. hásta-; Gk. kheir; L. praesto “at hand;” Arm. jern “hand;” Lith. pa-žastis “arm-pit;” PIE *ghes-to-) + rasi, verbal noun of rasidan “to attain; to arrive” (Mid.Pers. rasidan “to arrive, to mature;” O.Pers./Av. rasa- present stem of ar- “to move, go or come toward;” cf. Skt. ar-, rcchati “reaches;”
Gk. erkhomai “to go, to reach;” L. oriri “to rise”);
dâštan “to have, hold, maintain, possess” (Mid.Pers. dâštan; O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind;” cf.
Skt. dhr-, dharma- “law;”
Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support”).

An unexpected and undesirable event; an unforeseen and unplanned event or circumstance.

Etymology (EN): M.E., from M.Fr., from L. accidentum, pr.p. of accidere “to happen, fall out” from ad- “to” + cadere “to fall.”

Etymology (PE): Došâmad, from doš- prefix meaning “bad, ill, abnormal” (Mid.Pers. duš-, duž-, O.Pers./Av. duš-, duž- “bad, ugly, evil,” cognate with
Skt. dus-, Gk. → dys- “bad, hard, unlucky”)

• âmad, from âmadan “to come, happen,” as in the current term pišâmad “event, happening.”

1. To welcome or salute with shouts or sounds of joy and approval; applaud.
   

2. To announce or proclaim with enthusiastic approval (Dictionary.com).

See also: From ac-, variant of → ad-, + → claim.

To bring to its goal or conclusion; carry out; perform; finish (Dictionary.com).

Etymology (EN): M.E. accomplice, from O.Fr. acompliss-, p.p. stem of acomplir “to fulfill, complete,” from L. → ad- “to” + complere “to fill up,” → complete.

Etymology (PE): Anjâm dâdan, literally “to make an end of, to finish; to carry out,” from anjâm “end, conclusion” (from Mid.Pers. hanjâm “end, completion, outcome;” ultimately
from Proto-Ir. *ham-gam-, from *ham-, → com-,

• *gam- “to come,” cf. Av. gam- “to come,” related to Pers. âmadan, → walk), + dâdan “to give, commit,” → yield.

1. An act or instance of carrying into effect; fulfillment.
   

2. Something done admirably or creditably (Dictionary.com).

See also: → accomplish; → -ment.

On a multiuser → computer system or → network, a method of identifying a particular → user, usually requiring a → password to enter.

Etymology (EN): M.E. a(c)ount(e), ac(c)ompte, from O.Fr. aco(u)nte, acont, “account, reckoning, terminal payment,” from a “to,” → ad- + cont “counting, reckoning of money to be paid,” from L.L. computus “a calculation,” from L. computare “calculate, → compute.”

Etymology (PE): Ažmar, from Kurd. žmârtin, žmêr-, variant of šomâr, šomârdan “to → count.”

To grow or increase gradually, by the process of → accretion.

See also: Back-formation from → accretion.

The gas involved in various accretion processes, such as that fed into an → accretion disk, pulled by a compact object, or used in the mass growth of a galaxy.

See also: → accretion; → gas.

That → accretes, such as
→ accreting star and → accreting neutron star.

See also: Adjective from → accrete + → -ing.

A → neutron star in a → binary system that accretes matter from the → campion star, either from the → stellar wind or from an → accretion disk that forms if the companion overflows its → Roche lobe. The → gravitational energy from the infalling matter provides at least part of the energy for the observed radiation and the accretion torques dominate the spin evolution. Despite these common properties, accreting → neutron stars display a wide variety of behaviors, depending on the neutron star → magnetic field strength,
mass of the companion and properties of → accretion
(A. K. Harding, 2013, Front. Phys. 8, 679).

See also: → accreting; → neutron; → star.

The star which → accretes matter, particularly in its protostellar phase or in a close binary system.

See also: → accreting; → star.

1. The process by which an object increases its mass under the influence of its
   → gravitational attraction.
   Accretion plays a key role in a wide range of astrophysical phenomena. In particular stars result from the accretion of material by a → protostar from a surrounding → molecular cloud. The accumulation of mass on the protostar involves the formation of an → accretion disk.
   Theoretical and observational investigations of protostars and newborn stars indicate the important role of → magnetic fields in this process. They favor the magnetospheric accretion model for mass transfer from the circumstellar disk onto the newborn star. In this model, the stellar magnetosphere → truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along the magnetic field lines and hits the stellar surface at approximately the → free fall velocity, causing a strong accretion shock. Various → emission lines, such as the hydrogen → Balmer series, He I 5876 Å, Brγ 2.17 μm, and so forth are formed in the infalling magnetospheric flow. Moreover, optical/ultraviolet excess continuum emission is produced in the → accretion shocks. The accretion is accompanied by mass ejection through collimated → bipolar jets.
   

2. Accumulation of dust and gas onto larger bodies by → coalescence under the influence of their mutual → gravitational attraction or as a result of chance collisions.
   

See also:
→ accretion column, → accretion disk, → accretion flow, → accretion rate, → accretion shock, → accretion time.

Etymology (EN): L. accretionem (nom. accretio, gen. accretionis) “a growing larger,” from stem of accrescere, from ad- “to” + crescere “to grow”.

Etymology (PE): Farbâl from prefix → far- which conveys “increase, abundance” + bâl, from bâlidan “to grow, to wax great,” Mid.Pers. vâlitan, Av. varəd-, varədait- “to increase, augment, strengthen, cause to prosper,” Skt. vrdh-, vardhati.

The channel through which matter is accreted onto a body such as a → protostar, → white dwarf, → neutron star, or → black hole.
The accreting body possesses magnetic fields strong enough to disrupt the → accretion flow and carry the material through column-shaped channels directly on to a small fraction of the stellar surface near the magnetic poles.

See also: → accretion; → column.

A rotating disk of gas and dust formed around a center of strong gravity that pulls material off a surrounding or near-by gaseous object. Accretion disks are associated with several astrophysical objects such as → binary stars, → protostars, → white dwarfs, → neutron stars, and → black holes. Accretion disk forms because the infalling gas does not directly crash the accreting object due to its too high → angular momentum. The individual particles go into a circular orbit around the accretor because the circular orbit has the lowest energy for a given angular momentum. A spread in angular momentum values will give a population of particles moving on different orbits, so that a rotating disk of matter forms around the object.
The matter in the disk becomes very hot due to internal friction and
→ viscosity as well as the tug of the accreting object. Since this hot gas is being accelerated it radiates energy and loses angular momentum and falls onto the accretor. Theoretical and observational pieces of evidence point to the importance of → magnetic fields in the accretion process. According to current models, the stellar magnetosphere → truncates the disk at a few stellar radii. Gas from the disk accretes onto the star along the magnetic field lines and hits the stellar surface at approximately the → free fall velocity, causing a strong accretion shock. See also → flared disk, → self-shadowed disk, → protoplanetary disk, → alpha disk model.

See also: → accretion; → disk.

1. Flow of matter during an accretion process.
   

2. In a → binary system, flow of matter
   from the losing-mass → companion toward the compact one. The flow can be from a → stellar wind or through the → inner Lagrangian point.
   

3. → cold accretion flow, → hot accretion flow.

See also: → accretion; → flow.

The amount of mass → accreted during unit time. The accretion rate for the → collapse of a singular → isothermal sphere is expressed by:
dM/dt = 0.975 c_s³/G, where c_s is the isothermal → sound speed (Shu 1977, ApJ 214, 488). This relation can be written as:
dM/dt = 4.36 x 10^-6 (T / 20 K)^3/2 in units of solar masses per year, where T is the temperature. Observed temperatures of 10-20 K in regions of → low-mass star formation imply accretion rates of about 10^-6 to 10^-5 solar masses per year. Accretion rates for → massive stars amount to values of 10^-4 to 10^-3 solar masses per year.

See also: → accretion; → rate.

A → shock wave occurring at the surface of a compact object or dense region that is accreting matter with a → supersonic velocity from its environment. In the case of → young stellar objects the process is believed to take place by funneled streams in the form of → accretion columns that originate in the surrounding → accretion disk and flow along the → field lines of the → protostar → magnetosphere. The gas falls supersonically onto the surface of the central body and its impact
produces strong shocks of a few million → kelvin, a phenomenon that is observable in → X-rays.

See also: → accretion; → shock.

The time necessary for the → accretion of a definite amount of mass with a fixed → accretion rate.

See also: → accretion; → time.

An → astronomical object that accretes surrounding material. See also → accretion.

See also: Agent noun, from → accrete + → -or.

To gather or collect, often in gradual degrees; heap up (Dictionary.com).

See also: From ac-, → ad-, + → cumulate; → amass.

A device for storing electricity. An electric current is passed between two plates in a liquid. This causes charges (due to electrolysis) in the plates and the liquid. Same as → secondary cell.

Etymology (EN): From L. accumulator, from accumulare “to heap up,” from → ad- “in addition” + cumulare “heap up,” from cumulus “heap.”

Etymology (PE): Anbâré, from anbâr-, anbâštan, → amass; âkumgar, from âkumidan, → accumulate.

1. The state or quality of being → accurate.
   

2. The degree of nearness of a measured value to the standard or known value of the quantity, not to be confounded with → precision.

For example, a refrigerator holds a constant temperature of 5.0 °C. A thermometer is used seven times to read the temperature, with the following results: 6.4, 5.1, 6.3, 4.5, 5.3, 6.1, and 4.1. This distribution does not well match the actual temperature, therefore it lacks accuracy, and shows no tendency toward a particular value; it lacks precision, as well.

If the measured temperatures are 4.8, 5.3, 5.1, 5.0, 4.6, 5.2, and 5.0, the mean value is accurate, because it comes close to the actual temperature, but the distribution shows no clear tendency toward a particular value (lack of precision).

Now suppose that the measured temperatures are 6.2, 6.3, 6.1, 6.0, 6.1, 6.3, and 6.2. In this case every measurement is well off from the actual temperature (low accuracy), but the distribution does show a tendency toward a particular value (high precision).

Finally, if the measured temperatures are 5.0, 5.0, 4.8, 5.1, 5.0, 4.9, and 5.0, the distribution is very near the actual temperature each time (high accuracy), and does show a tendency toward a particular value (high precision).

Accuracy is often given to n → significant digits or n → decimal places.
For example e = 2.71828 … = 2.718 is rounded to two four significant figures or three decimal places. → accurate to n significant figures, → accurate to n decimal places.

Etymology (EN): From L. accuratus “prepared with care, exact,” p.p. of accurare “take care of,” from ad- “to” + curare “take care of.”

Etymology (PE): Rašmandi, from rašmand, from raš + adjective forming suffix -mand. Raš, from Av. root raz- “to right, correct, arrange;”
compare with Skt. raj “to reign, rule, direct,” Gk. oregein “to strech out,” L. rego “to direct, lead;” PIE *reg- “to move in a straight line.” Similarly, Av. râšta-, rašta- “straight,” Skt. rju “straight, right, upright,” Gk. orektos “elongated,” L. rectus “straight,” Ger. recht, E. right. In Mod.Pers. there are several derivatives:
râst, râšt (as in afrâšt(an)) “right; true,” rasté, rešté, raj, raž, râh, ris, râdé, radé, Lori rezg “row,” etc.

1. Conforming exactly to truth or to a standard; free from error.
   

2. Designating → accuracy.

See also: Accurate, from L. accuratus, → accuracy.

An expression specifying the number of meaningful digits to the right of the → decimal point. For example, e = 2.71828 … = 2.718 is said to be accurate to three decimal places and 2.72 to two decimal places.

See also: → accurate; → decimal; → place.

An expression specifying the number of meaningful digits used to express the value of a measured quantity. Same as accurate to n significant figures. For example, e = 2.71828 … = 2.718 is rounded to four significant digits, and 2.72 to three significant digits. → accurate to n decimal places.

See also: → accurate; → significant; → digit.

Able to be accused; open to an accusation (of); blameworthy, reprehensible (OxfordDictionaries.com).

See also: → accuse; → -able.

A charge or claim that someone has done something illegal or wrong; the action or process of accusing someone (OxfordDictionaries.com).

See also: → accuse; → -tion.

1. → accusative case.
   

   2. → accusatory.

Etymology (EN): M.E., from M.Fr., from L. accusativus, from ac-, → ad-, + -cusativus, combining form of causativus, → causative, a loan-translation of Gk. aitiatike, in the sense of pointing to the origin or cause, accusing.

Etymology (PE): 1) Kondâri, i.e. → objective case.
2) → accusatory.

The → grammatical case of a noun in some languages (such as Greek, Latin, G erman, Russian, Old Persian, Avestan, or Sanskrit), which shows that the noun is the → direct object of a → verb or a → preposition.

See also: → accusative; → case.

Containing an accusation; accusing (Dictionary.com).

See also: → accuse; → -tion.

To charge with the fault, offense, or crime (Dictionary.com).

Etymology (EN): M.E. ac(c)usen, from O.Fr. acuser “to accuse, indict, blame,” earlier “announce, report, disclose,” from L. accusare “to call to account,” from → ad- “to, toward, at, with regard to” + causari “give as a cause or motive,” from causa “reason,” → cause.

Etymology (PE): Mârzidan, from (Lâr, Gerâsh) mârz, (Farâmarzân) morz “blame, reproach, accusation,” maybe ultimately from Proto-Ir. *marc- “to destroy, damage;” cf. Av. mərənc- “to destroy,” (+ *para-) “to damage, injure,” (+ *ui-) “to ruin, spoil;” Mid.Pers. mwlncyn- / murnjên- “to destroy;” Khotanese mulch- “to cause to miscarry” (Cheung 2007).

A person or persons charged in a court of law with a crime, offense, etc.

See also: → accuse; → -tion.

A person who accuses, especially in a court of law.

See also: → accuse; → -tion.

The brightest star in the constellation → Eridanus. A → subgiant of → spectral type B5; apparent visual magnitude 0.5, about 140 → light-years distant (other names: HR 472, HD 10144). Recent interferometric observations show it to have a flattened shape imposed by fast rotation.

Etymology (EN): Achernar, from Ar. Axir an-Nahr “end of the river,” from axir “end” + nahr “river”.

Etymology (PE): Âxer-e nahr, from Axir an-Nahr.
Rudpâyân “river’s end,” from rud “river,” → Eridanus, + pâyân “end”.

A class of → stony meteorites that lack → chondrules. They are made of rock that has crystallized from a molten state. Achondrites are relatively rare, accounting for about 8% of all meteorite falls.

See also: Achondrite, from Gk. prefix a- (an- before stems beginning with a vowel or h) “not, without, lacking” + Gk. chondrite, from chondr-, from chondros “grain,”

• affix -ite.

Of or relating to an optical system which is capable of transmitting light without decomposing it into constituent colors.

See also: → a-; → chromatic.

Lens (or combination of lenses) that brings different wavelengths within a ray of light to a single focus, thus overcoming chromatic aberration.

See also: → achromatic; → lens.

A substance that releases hydrogen ions to form a solution with a pH of less than 7, reacts with a base to form a salt, and turns blue litmus red.

See also: From Fr. acide, from L. acidus “sour,” adj. of state from acere “to be sour,” acer “sharp, pungent, bitter;” from PIE base *ak- “sharp, pointed.”

Being or containing an acid; of a solution having an excess of hydrogen atoms (having a → pH of less than 7).

See also: → acid; → -ic.

1. To admit to be real or true; recognize the existence, truth, or fact of.
   

   2. To recognize the authority, validity, or claims of.
      

   3. To show or express appreciation or gratitude for (Dictionary.com).

Etymology (EN): A blend of M.E aknow “admit or show one’s knowledge” (from O.E. oncnawan “understand”) and M.E. knowlechen “to admit.”

Etymology (PE): Âdânidan, from âdân, from prefix â- + dân present stem of dânestan, → know; cf. Sogd. âzân, azân “to acknowledge, to confess,” from prefixed zân, variant of dân.

1. Recognition of the existence or truth of something.
   

   2. An expression of appreciation (Dictionary.com).

See also: → acknowledge; → -ment.

Pertaining to the sense of hearing, or to → sound waves.

See also: → acoustics.

One of several peaks appearing in the → CMB angular power spectrum of the → cosmic microwave background radiation which are ripples left by acoustic oscillations of the plasma-radiation fluid in the early Universe (→ baryon acoustic oscillations). When the Universe was small and very hot, the free electron density was so high that photons could not propagate freely without being scattered by electrons. Ionized matter, electrons and radiation formed a single fluid, with the inertia provided by the baryons and the radiation pressure given by the photons.

See also: → acoustic; → peak.

Same as → sound pressure.

See also: → acoustic; → pressure.

A type of → longitudinal wave that consists of mechanical → vibrations of small
→ amplitude propagated in an → elastic medium. Acoustic waves exhibit phenomena like → diffraction, → reflection, and → interference, but not → polarization. Also called → sonic and → sound waves. See also
→ acoustic wave equation. The branch of physics concerned with the properties of sonic waves is called → acoustics.

See also: → acoustic; → wave.

A → differential equation that describes the time evolution of the → scalar potential of the field φ. It is expressed by: ∇²φ = (1/c²)∂²φ/∂t², where c is → velocity of → longitudinal waves and ∇² is the → Laplacian operator.

See also: → acoustic; → wave; → equation.

1. The study of sound, especially of its generation, propagation, and reception.
   
2. Those qualities of an enclosure that together determine its character with respect to distinct hearing.

Etymology (EN): From Fr. acoustique, from Gk. akoustikos “pertaining to hearing,” from akoustos “heard, audible,” from akouein “to hear,” from copulative prefix a- + koein “to mark, perceive, hear,” from PIE root *(s)keu- “to notice, observe.”

Etymology (PE): Sedâyik from sedâ “sound” + Pers. suffix -ik, → -ics. Sedâ is most probably Persian,
since it exists also in Indo-Aryan languages: Skt. (late Vedic): sabda “articulate sound, noise,”
Pali and Prakriti: sadda “sound, noise,” Sindhi: sadu, sado “shout, call,” Gujrâti sad “call, voice, echo,” Marathi: sad “shouting to,” Konkani sad “sound,” Sinhali: sada “sound.” Therefore, sadâ in Arabic “reverbrating noise, echo” may be borrowed from Persian, or a coincidence. Note that for the author of the classical Persian dictionary Borhân-e Qâte’ (India, 1652 A.D.), the Arabic term is a loanword from Persian.

See also: → phone; →phonetics.

Relating to or occurring at sunset. → heliacal

Etymology (EN): Acronical, from Gk. akronukos, from akros
“tip, end,” cf. Av. aγra- “top, first,” Skt. agra- “first, foremost, climax”

• nuks, nuktos “night,” → night.

Etymology (PE): Šâmgâhi, adj. of šâmgâh “evening,” from šâm “evening, evening meal” + gâh “time.” The first component, šâm, from Mid.Pers. šâm “evening meal, supper,” from Av. xšāfnya- “evening meal,” from Av. xšap-, xšapā-, xšapan-, xšafn- “night” (O.Pers. xšap- “night,” Mid.Pers. šap, Mod.Pers. šab “night”); cf. Skt. ksap- “nigh, darkness;” Hittite ispant- “night.” The second component gâh “time,” Mid.Pers. gâh, gâs “time,”
O.Pers. gāθu-, Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”

The rising of a star in the sky at or just after sunset. → heliacal rising.

See also: → acronical; → rising.

The setting of a star at nightfall. → heliacal setting.

See also: → acronical; → setting.

A southern hemisphere bright star (α Crucis) of magnitude 0.77 lying at a distance of 321 → light-years. It is apparently made up of three components. The primary component, Acrux A, is a blue subgiant of apparent magnitude 1.34 and spectral type B0.5 IV. The B component lies at about 4.1 arcsec away from the A component, which represents a distance of at least 400 AU, that is, more than 10 times the distance between the sun and planet Pluto. The C component lies about 90 arcsec away from A.

Etymology (EN): Acrux, from A, from Alpha, designating the brightest star of the constellation + crux, the constellation name. The name Acrux is probably a coinage of the American astronomer, Elijah H. Burritt, who published several editions of an astronomical atlas between 1833 and 1856.

1. The process of doing or performing something;
   something done or performed.
   

2. (v.intr.) To carry out an action; to produce an effect.

Etymology (EN): Act, from O.Fr. acte, from L. actus “a doing” and actum “a thing done,” both from agere “to do, set in motion, drive,
urge, chase, stir up,” from PIE root *ag- “to drive, draw out or
forth, move” (cf. Gk. agein “to lead, guide, drive, carry off,”
agon “assembly, contest in the games,” agogos “leader;” Av. az- “to drive (away),” azaiti “drives,”
Mod.Pers. govâz “stick for driving cattle,” from Av. gauuāza-, from gao- “cow, ox, cattle” (→ Bootes)

• āza-, from az-, as above; Skt. aj- “to drive, sling,” ájati “drives,” ajirá- “agile, quick.” The E. agile “characterized by quickness, lightness, and ease of movement;
  mentally quick or alert” is from this root.

Etymology (PE): In major European languages there are two fundamental and very close verbs which convey “work, action, activity”. These are: 1) to do (in French faire, Spanish hacer, German machen) and
2) to act (French agir, Spanish actuar, German
handeln). In Persian there is only one word for these two concepts:
kardan; and this is obviously a big handicap.
An ad hoc equivalent for action has therefore been
koneš, from kardan “to do.” The problem is that
this solution, despite being widely used, confounds “to do” with
“to act,” and is incapable of forming all the related derivatives.
Therefore, we propose žir, which derives from Av. žirā- “active, agile, clever;” Mid.Pers. žir, zir “active, busy” (loaned in Arm. žir “active, busy, clear”), Mod.Pers. zirak “clever, alert, intelligent;” Kurd. žir “agile,” žiri “agility.”

Any member of the group of → chemical elements with → atomic numbers from 89 (→ actinium) to 103 (→ lawrencium), analogous to the → lanthanides.

See also: From the chemical element → actinium.

A silver-white radioactive → chemical element; symbol Ac. The first member of the → actinide series of the → periodic table. → Atomic number 89; → atomic weight 227.0278; → melting point about 1,050°C; → boiling point 3,200°C ± 300°C; → specific gravity 10.07; → valence +3. It is found with uranium minerals in pitchblende. Its longest lived → isotope is ²²⁷Ac with a → half-life of 21.77 years.

See also: From actin-, variant of actino-, from Gk. aktinos “ray, beam” + → -ium. The discovery of actinium is shared between two chemists who independently found the element. The earlier discovery was made by the French chemist André Debierne (1874-1949) in 1899 in pitchblende residues left after Pierre and Marie Curie had extracted → radium. The element was rediscovered in 1902 by the German chemist Friedrich Otto Giesel (1852-1927), who called it emanium.

Any instrument for measuring the intensity of radiation, especially that of the Sun, in its thermal, chemical, and luminous aspects.

Etymology (EN): Actinometer, from actino- combining form with the meaning “ray, beam,” from Gk. aktis, aktin
“ray,” + → -meter.

Etymology (PE): Partow, → ray; + -sanj, → -meter.

1. The process or state of acting or of being active.
   

2. According to → Newton’s third law of motion, an external force that is applied to a body and that is counteracted by an equal force in the opposite direction ( → reaction).
   

3. A quantity whose → dimension (ML²T^-1) coincides with that of → angular momentum, the → impulse of a force, or → energy x → time. The action plays an important part in → analytical mechanics,
   → quantum mechanics, and in a number of other fields of physics. Initially introduced in analytical mechanics, the concept of action has become a basic ingredient of modern physics, due
   to the role it has played in the generalization of → variational principle.
   

4. A scalar quantity computed as a function of the path followed by a system during its evolution between an initial instant t_i and a final instant t_f. It is defined by the → integral of the → Lagrangian between the two instants:
   S = ∫L dt
   

In the framework of the → field theory, the action is expressed by the integral of the → Lagrangian density over the corresponding space-time volume:
S = ∫L_d d⁴x.

In classical physics, the path actually followed by the system is the one for which S is stationary (→ least action problem).

5. → quantum of action.
   

6. Math.: The action is a → functional, a mathematical relationship which takes an entire path and produces a single number.

Etymology (EN): Action, from O.Fr. action, from L. actionem, from agere “to do,” → act.

Etymology (PE): Žireš, verbal noun from žir stem of žiridan “to act;” → act. Koneš, noun from kardan “to do, to make,” Mid.Pers. kardan, O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “makes,” cf. Skt. kr- “to do, to make,” krnoti “makes,”
karma “act, deed;” PIE base k^wer- “to do, to make.”

The instantaneous action of a body on another body independently of the distance separating them. The description of → gravity by → Newton’s law and → electrostatics by → Coulomb’s law are examples of action at a distance. According to Newton, → gravitation acts directly and instantaneously between two objects. For example, if the Sun should suddenly break apart, the Earth’s orbit would be affected instantaneously. However, action at a distance violates the → principle of relativistic causality. According to → general relativity, gravitational effects
travel at the → speed of light. For modern physics there is no instantaneous action at a distance.

See also: → action; → distance.

The time integral associated with the evolution of a physical system in the phase space.

See also: → action; → variable.

1. To induce activity in a system that is static, as in neutron activation of radioactivity.
   
2. To start the operation of an electrical device.

Etymology (EN): Activate, verb from → active.

Etymology (PE): Žirândan, transitive verb from žir,
→ act.

1. The process of inducing or creating a state of → activity.
   

2. The process of producing a → radioactive isotope by bombarding a → stable → nuclide with → nuclear particles (such as → protons, → neutrons, → alpha particles, heavy ions, etc.).

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

Chemistry: The minimum amount of energy that is required to activate → atoms or → molecules to a condition in which they can undergo a → chemical reaction. Most reactions involving neutral molecules cannot take place at all until they have acquired the energy needed to stretch, bend, or otherwise distort one or more → bonds. In most cases, the activation energy is supplied by → thermal energy.

See also: → activation; → energy.

1. Being in a state of action; not quiescent.
   

2. Functioning or capable of functioning.
   

3. Marked by vigorous activity.

Etymology (EN): M.Fr. actif, from L. activus, from actus, p.p. of agere, → act.

Etymology (PE): Žirâ, adj. from stem žir, → act + suffix -â.

A central region of an → active galaxy, which is
a → light-year or less in diameter and has an abnormally high luminosity. The nucleus emits high energy radiation (→ gamma rays, → X-rays, → ultraviolet) and shows → variability over various time-scales, sometimes very short (hours to weeks). Emission line spectra reveal high velocity motions up to 10⁴ km s^-1. AGNs are divided into two main types. Type I refers to an AGN whose nucleus is visible (the spectra has both narrow and broad emission lines), while in type II AGN, the broad line region (BLR) is obscured and the lines are very narrow. This may be due either to the viewing angle or some intrinsic difference in structure. See also → broad-line region, → narrow-line region, → quasar.

See also: → active; → galactic;
→ nucleus.

A galaxy that produces huge amounts of energy at its center, which cannot be attributed to normal processes from stars, interstellar medium, and their interactions. There are several types of active galaxies: → Seyfert galaxies, → quasars, and → blazars. All of these objects show brightness variations, some as short as 3 hours. These fluctuations indicate a relatively very small size for the central object,
because an object cannot vary in brightness faster than light can travel across it. For example, an object that is one → light-year in diameter cannot vary significantly in brightness over a period of less than one year.

See also: → active; → galaxy.

A technique for improving the → resolving power of a telescope by controlling the shape of the main mirror at a relatively slow rate. The → image quality is optimized automatically through
constant adjustments by in-built corrective → actuators operating at fairly low temporal frequency ~0.05 Hz or less. → adaptive optics.

See also: → active; → optics.

A solar → prominence with very rapid motion (up to 2,000 km s^-1), moving and changing in appearance over a few minutes of time, in contrast to a → quiescent prominence.

See also: → active; → prominence.

An area of the Sun exhibiting → solar activity with the presence of → sunspots, → flares,
→ faculae, → prominences, and other phenomena associated with intense magnetic fields.

See also: → active; → region.

The Sun during its 11-year cycle of activity when spots, flares, prominences, and variations in radiofrequency radiation are at a maximum.

See also: → active; → sun.

1. The doctrine or practice of vigorous action or involvement as a means of achieving political or other goals, sometimes by demonstrations, protests, etc. (Dictionary.com). See also → activist.
   

2. Philosophy: a) A theory that the essence of reality is pure activity, especially spiritual activity, or process.
   b) A theory that the relationship between the mind and the objects of perception depends upon the action of the mind (Dictionary.com).

See also: → active; → action; → -ism.

An especially active, vigorous advocate of a cause, especially a political cause (Dictionary.com).

See also: Agent noun from → activism.

For a radioactive substance, the average number of atoms disintegrating per unit time.

Etymology (EN): Activity, from → active + -ity.

Etymology (PE): Žirandegi, noun from žirandé, → active.

1. One who takes part; a participant.
   

   2. A theatrical performer.

See also: → act + → -or.

1. Existing in act or fact; real.
   

2. Philosophy: The character of what is realized in act (such as a statue), as opposed to what can potentially be realized (from a piece of rock).

Etymology (EN): M.E. actuel, from O.Fr. actuel “now existing, up to date,” from L. actualis “active, pertaining to action,” adj. form of actus, → act.

Etymology (PE): Žirvand, from žir, → act, + -vand a suffix forming adjectives denoting “possessed of, similarity, relation,” variants -âvand, -van (e.g. xodâvand “possessor, master,” pulâdvand “hard as steel, name of a hero,” dowlatvand “rich,” setarvan “mule-like, barren,” polvan “bridge-like, a raised path”), from Mid.Pers. -âwand (hunarâwand “skilled,” warzâwand “powerful”); Av. -vant (aurvant- “swift,” surunvant- “audible”); cf. Skt. -vant (amavant- “having attacking power”).

1. The quality or state of being actual or something that is actual; reality.
   

2. Philosophy: According to Aristotle, what has → form and can causally interact with other things, as opposed to → potentiality.

See also: → actual; → -ity.

The act or process of actualizing.

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

To make actual or real; turn into action or fact.

See also: → actual; → -ize.

1. To put into mechanical action or motion.
   

2. To incite or move to action; motivate.

Etymology (EN): From L. actuatus, p.p. of actuare, from actus, → act.

Etymology (PE): Baržiridan, from bar- “on, upon, up,” → on-, + žiridan “to → act.”

The act or process of putting into action; activation.

See also: Verbal noun of → actuate.

A mechanism to activate process control equipment by use of pneumatic, hydraulic, or electronic signals.

See also: → actuate; → -or.

Sharpness; acuteness; keenness of perception.

Etymology (EN): M.E., from M.Fr. acuité, from M.L. acuitatem (nom. acuitas) “sharpness,” from L. acus “needle,” acuere “to sharpen,” from PIE root *ak- “be sharp.”

Etymology (PE): Tiznâ “sharpness,” from tiz, “→ sharp,”

• -nâ, a suffix that transforms adjective into noun (compare with tangnâ, derâznâ, farâxnâ, etc.).

The ability of the → eye to see separately two points close to each other. It is a measure of the → resolving power of the eye’s → optical system and depends on the density of cells in the → retina. The maximum acuity of the normal human eye is around 0.5 minutes of arc.

See also: → acuity; → vision.

For the specific purpose at hand, as opposed to a general solution; also, by extension, improvised or impromptu.

Etymology (EN): From L. ad “to, with, in,” cf. Skt. adhi “near,” PIE *ad- “to, near, at.”

Etymology (PE): Pad im, from Mid.Pers. pad “to, at, for, in” (Mod.Pers. bé “to, for, in, on, with, by”);
O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of;” Av. paiti “to, toward, in, at;” cf.
Skt. práti “toward, against, again, back, in return, opposite;” Pali pati-; Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti)

• Mid.Pers. im “this;” from Old.Pers./Av. ima “this;” Skt. imá; cf. Lori (Laki) im “this side.” The Mid.Pers. im occurs in Mod.Pers. as em- in emruz “today,” emšab “tonight,” and emsâl “this year.”

Addition of adjustments to a theory to save it from being falsified by compensating for anomalies not anticipated by the theory in its unmodified form. Theories that rely on continual, ad hoc adjustments are distrusted.

See also: → ad hoc; → hypothesis.

A fallacious objection to an argument or factual claim by appealing to a characteristic or belief of the person making the argument or claim, rather than by addressing the substance of the argument or producing evidence against the claim (wiktionary).

Etymology (EN): Literally “to a man,” from → ad- “to,” + hominem, accusative of homo “man,” → human.

Etymology (PE): Pad mart, literally “to a man,” from pad “to,” → ad hoc, + mart, → man.

Prefix meaning “to, toward, addition to, near,” from L. ad “to, toward.” It is modified to ac- or af- or ag- or al- etc. according to the following consonant.

Etymology (EN): Cognate with E. at, from O.E. æt “near, by, at”; compare with O.N., Goth. at, O.Fris. et, O.H.G. az, Skt. adhi “near,” PIE *ad- “to, near, at”.

To make suitable to or fit for a specific use or situation.

Etymology (EN): From M.Fr. adapter, from L. adaptare “adjust,” from ad- “to” + aptare “join,” from aptus “fitted.”

Etymology (PE): Niyâvidan from Mid. Pers. niyâw “apt, suitable, appropriate” + -idan verb making suffix.

Capable of adapting or of being adapted.

See also: → adapt + → -able.

1. The act or process of adapting.
   
2. The state of being adapted.

See also: → adapt.

Physiological process whereby the eye adjusts its sensitivity for different levels of illumination.

See also: → adaptation, → eye.

1. General: One that adapts.
   An appliance for connecting objects of different sizes or designs in an apparatus.
   
2. Astro.: A device that allows an observing instrument (imaging camera, spectrograph) to be mounted on a focus (e.g. Cassegrain) of a telescope.

See also: → adapt + → -er.

Same as → adaptation.

See also: → adaptation.

1. Relating to or exhibiting adaptation.
   
2. Capable of adapting or of being adapted. → adaptive optics; → adaptive optics system; → extreme adaptive optics.

See also: → adapt; → -ive.

A type of → algorithm that dynamically achieves high → resolution in localized regions of multidimensional → numerical simulations. AMR provides a higher → accuracy solution at lower costs, through an automatically → optimal distribution of → grid points for the computation region. It relies on locally refined mesh or mesh patches to increase the resolution of an underlying coarse mesh only where needed. It can alleviate some of the complexities of the generation of high quality grid and reduce the number of → iterations of “trial-and-error” between the grid generation and solution required for tailoring the grid to the specification of a problem. Thus, it can offer orders of magnitude saving in computational and storage costs over an equivalent uniformly refined mesh. AMR was originally developed for → inviscid, → compressible flow (Berger et al., 1984, Adaptive Mesh Refinement for Hyperbolic Partial Differential Equations. J. Comp. Phy., 53, 484). It has been extended to solve → Navier-Stokes equations, time dependent problems and more. Several AMR techniques have been developed and applied to compressible flow fields to capture characteristics at the strong gradient or discontinuous regions requiring higher space resolution, such as regions involving → shock waves, vortices (→ vortex), and → wakes
(see, e.g., Qingluan Xue, “Development of Adaptive Mesh Refinement Scheme and Conjugate Heat Transfer Model for Engine Simulations” (2009), Iowa State Univ., Graduate Theses and Dissertations, Paper 10678).

See also → Smoothed Particle Hydrodynamics.

See also: → adaptive; → mesh; → refinement.

A technique for improving the → image quality of a telescope against → atmospheric turbulence in which image distortions are compensated by high-speed changes in the shape of a small, thin mirror. → wavefront; → wavefront distortion; → wavefront correction; → Strehl ratio; → tip-tilt mirror, → Shack-Hartmann wavefront sensor, → active optics.

See also: → adaptive; → optics.

An → adaptive optics instrument used on the → European Southern Observatory (ESO) 3.6-m telescope at La Silla. It was an upgraded version of COME-ON-PLUS, the → Very Large Telescope (VLT) adaptive optics prototype.

It had 52 → actuators and performed corrections of the mirror 200 times per second. The reference → wavefront was sensed in the → visible. The observation was done in the → near-infrared (1-5 μm).

See also: → adaptive; → optics; → near-infrared; → system.

An → optical system that uses → adaptive optics.

See also: → adaptive; → optics; → system.

1. To unite or so as to bring about an increase.
   

2. To increase a number by another number using addition, which leads to a → sum.

Etymology (EN): M.E. adden, from L. addere “add to, join,” from → ad- “to” + -dere combining form meaning “to put, place,” from dare “to give, grant,” from PIE base *do- “to give” (cf. Av. and O.Pers. dâ- “to give, grant, yield,” Av.
dadâiti “he gives,” Skt. dadâti “he gives,” Gk. didomi “I give,” L. do “I give”).

Etymology (PE): Bardâyidan, from bar- “on; up; upon; in, into; at; forth,” → on-, + O.Pers./Av. dā- “to give, grant, put,” dadāiti “he gives;” Mid.Pers./Mod.Pers. dâdan “to give, put”
(cf. Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”)

• -y-, epenthetic vowel, + infinitive suffix -idan.
  Afzudan, → increase.

A number or quantity to be added to another. In the expression 5 + 3, both 5 and 3 are addends.

Etymology (EN): From L. addendus “that which is to be added,” → add.

Etymology (PE): Bardâyé, from present stem bardây-,
→ add, + noun forming suffix -é.

The operation of combining two or more quantities to obtain a third quantity called their → sum. The result of adding.

See also: Verbal noun of → add.

The → plus sign +. It is believed to be a shortened form of the letters e and t in the L. word et, which, in early German manuscripts was the term for addition. The signs + and - were first used by Johann Wiedmann in 1489.

See also: → addition; → sign.

1. Involving → addition.
   

2. Something added to a substance to alter or improve it.

Etymology (EN): From L. additivus “added, annexed,” from p.p. stem of addere, → add; → -ive.

Etymology (PE): 1) Related to bardâyeš, → addition; 2) from bardây- prsent stem of bardâyidan, → add,

• -é nuance suffix.

Color created by mixing light rays of different colors. Combining all the color rays of light results in white light.
See also → subtractive color.

See also: → additive; → color.

The number which can be added to any other number without changing the magnitude of that number: zero. → multiplicative identity.

See also: → additive; → identity.

If E₁, E₂, …, E_n
are n → mutually exclusive events, then the probability of occurrence of at least one of them is the sum of their individual probabilities: P(E₁ + E₂ + … + E_n) = P(E₁) + P(E₂) + … + P(E_n).

See also: → additive; → law; → probability.

A binary star, in the constellation → Canis Major, 470 → light-years distant from Earth. The main star possesses an apparent magnitude of +1.5 and belongs to the spectral classification B2 II. The +7.5 magnitude companion star is 7’’.5 apart from the main star.

Etymology (EN): Adhara, from Ar. adhârâ “virgins,” plural of adhrâ’ “virgin”.

Etymology (PE): Azârâ, from Ar. Adhara.

1. To stay attached; stick fast; cling.
   

2. Physics: (of two or more dissimilar substances) to be united by a molecular force acting in the area of contact (→ adhesion).

Etymology (EN): From M.Fr. adhérer or directly from L. adhaerere “to stick to,” from → ad- + haerere “to stick.”

Etymology (PE): Âdusidan, from intensive/nuance prefix â-

• dusidan (Dehxodâ) “to stick, to adhere,” maybe related to Proto-Ir. *dauc- “to sew;” Pers. duxtan, duz- “to sew.”

1. The act or state of adhering; adhesion.
   

2. The quality of adhering; steady devotion, support, or attachment (Dictionary.com).

See also: → adhere + -ence, → -ance.

1. A person who follows or upholds a leader, cause, etc.; supporter; follower.
   

2. Sticking; clinging; adhering (Dictionary.com).

Etymology (EN): From O.Fr. adherent or directly from L. adhaerentem pr.p. of adhaerere “to stick to,” → adhere.

Etymology (PE): Âdusandé, from âdusidan, → adhere.

1. The act or state of adhering; state of being adhered or united.
   

2. Steady or devoted attachment, support, etc.; adherence.
   

3. Physics: The molecular force of attraction in the area of contact between unlike bodies that acts to hold them together (Dictionary.com). See also → cohesion.

See also: Verbal noun from → adhere; → -tion.

1. Of or pertaining to the molecular force → adhesion.
   

2. A substance used for sticking objects or materials together; glue.

See also: Adjective from → adhere.

The force of → attraction between molecules of different substances; for example, the force between the molecules of a solid and a liquid. When water is poured on clean glass, it tends to spread, forming a thin, uniform film over the surface. This is because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the glass, thus avoiding the repulsion between like molecules.

See also: → adhesive; → force.

A → late-type → giant star of → spectral type K0-IIIb in → Andromeda. Other designations: ξ
(Xi) Andromedae, HR 390, and HD 8207. → Visual magnitude = 4.88, B - V = +1.08.

See also: From Ar. Al-dhayl “skirt of a garment; tail.”

A → thermodynamic process that occurs without → loss or → gain of → heat.

See also:

→ adiabatic change, → adiabatic index, → adiabatic initial conditions, → adiabatic phase, → adiabatic process, → adiabatic shock, → adiabatic temperature gradient, → superadiabatic temperature gradient.

Etymology (EN): From Gk. adiabatos “impassable,” from a- “not” + diabatos “passable,” from diabainein “to go across,” from dia- " through" + bainein “to go.”

Etymology (PE): Bidarrow from bi- “not; without” + darrow “way out,” from dar “out” + row “to go, going,” from raftan “to go.”

A change taking place in a system that has perfect thermal insulation, so that heat cannot enter or leave the system and energy can only be transferred by work.

See also: → adiabatic; → change.

Of a gas, the ratio of its → specific heat at constant pressure to its specific heat at constant volume: γ = C_P/ C_V.

See also: → adiabatic; → index.

The assumption whereby the density fluctuations in the very → early Universe would be produced by compressing or decompressing of all components of a homogeneous Universe. The adiabatic initial conditions lead to coherent oscillations in the form of peaks in the → temperature anisotropy spectrum. See also → acoustic peak, → baryon acoustic oscillation.

See also: → adiabatic; → initial; → condition.

Same as the → Sedov-Taylor phase.

See also: → adiabatic; → phase.

A → thermodynamic process in which no → heat is supplied to or rejected from a system. → polytropic process.

See also: → adiabatic, → process.

A → shock wave without → radiative cooling. The term “adiabatic” refers to the fact that no → heat is removed during shock.

See also: → adiabatic; → shock.

The temperature gradient defining the → radiative equilibrium
condition in a region. It is expressed as: dT/dr = (1 - 1/ γ)((T / P)(dP / dr), where T and P are temperature and pressure, dT / dr and dP / dr temperature and pressure gradients respectively, and γ = C_P / C_V.
For radiative equilibrium to be stable against → convection, the actual temperature gradient must be less than the adiabatic temperature gradient, i.e. |dT /dr|_rad < |dT /dr|_ad. See also → Schwarzschild’s criterion.

See also: → adiabatic; → temperature; → gradient.

In grammar, a word that qualifies, describes, or quantifies a noun.

Etymology (EN): M.E., from O.Fr. adjectif, from L. adjectivum “that is added to (the noun),” neuter of adjectivus “added,” from p.p. of adicere “to throw or place (a thing) near,” from → ad- “to” + iacere “to throw,” → jet.

Etymology (PE): Zâbé, from zâb “attribute, quality” (Dehxodâ); probably related to zib “beauty, adornment,” zibâ “beautiful, adorned,” zivar “ornament,” zab “easy; gratis; right, direct;” from Proto-Ir. *zai- “to adorn, to equip.”

1. To be close to or in contact with.
   

2. To attach or append (dictionary.com).

Etymology (EN): M.E., from O.Fr. ajoindre“join together, unite,” from L. adjungere “fasten on, harness, join to,” from → ad- “to” + jungere “to bind together,” cognate with → yoke.

Etymology (PE): Âbandidan, âbanidan, from prefix â- + band, vand, bastan, in dialects contracted to ban-, van- “to bind, attach,” → band,

• -idan infinitive suffix.

Literally “joined to.” → adjoint matrix.

See also: → adjoin.

The → transpose of a → matrix in which each → element is replaced by its → cofactor. Same as → conjugate transpose and → Hermitian conjugate.

See also: → adjoint; → matrix.

1. To change (something) so that it fits, corresponds, or conforms; adapt; accommodate.
   

2. To put in good working order; regulate; bring to a proper state or position (Dictionary.com).

Etymology (EN): M.E. ajusten, “to correct, remedy; arrange, settle, compose,” from M.Fr. adjuster, O.Fr. ajouter “to join,” from L.L. adjuxtare “to bring near,” from L. → ad- “to” + juxta “next,” related to jungere “to join,” from PIE *yeug- “to join,” → conjugate, akin to E. → yoke.

Etymology (PE): Barjutidan, from prefix bar-, → on-,

• Kurd. Soriani jut, jot “pair, couple, twin,” Aftari jot “yoke,” classical Pers. yuq, → yoke.

The act of adjusting.

See also: Verbal noun of → adjust.

1. To choose or take as one’s own; make one’s own by selection or assent.
   

2. To take and rear (the child of other parents) as one’s own child, specifically by a formal legal act.
   

3. To vote to accept (Dictionary.com).

Etymology (EN): M.E., from M.Fr. adopter, from L. adoptare, from → ad-

• optare, → opt.

Etymology (PE): Baroptidan, on the model of bargozidan “to choose,” from bar- “on, up, upon, in,” → on-,

• optidan, → opt.

The act of adopting. The state of being adopted.

See also: → adopt; → -tion.

Of or involving adoption. Acquired or related by adoption.

See also: → adopt; → -ive.

The second innermost known satellite of Jupiter, whose orbit is situated at a distance of about 129 000 km from the planet, and its orbital period is of 0.298 days; also known as Jupiter XV. Adrastea is 25 x 20 x 15 km in size.

Etymology (EN): In Gk. mythology, Adrastea was the daughter of Zeus and Ananke and the distributor of reward and punishments.

To take up and hold another substance on the surface.

See also: → adsorption.

1. A material that can hold or condense molecules of another substance on its surface by adsorption.
   
2. Relating to or capable of adsorption.

Etymology (EN): From → adsorb + →-ent.

Etymology (PE): Baršamandé, from baršamidan, → adsorb,

• -andé.

A process in which a layer of atoms or molecules of one substance forms on the surface of a solid or liquid. → absorption, → desorption, → sorption.

Etymology (EN): Adsorption from ad- “to” + sorption, from L. sorbere “to suck,” → absorption.

Etymology (PE): Baršam, from bar- “on, upon” + šam “to drink, sip,” → absorption.

1. A person who is fully grown or developed or of age.
   

2. Having attained full size and strength; grown up; mature. A person who has attained the age of maturity as specified by law (Dictionary.com).

Etymology (EN): From L. adultus “grown up, mature, adult, ripe,” p.p. of adolescere “to grow up, mature,” from → ad- “to” + alescere “be nourished,” from alere “to nourish.”

Etymology (PE): Bornâ, from Mid.Pers. purnây- “adult;” Av. pərənāyu- “adult, old;” (Baluchi warnâ “adolescent”), literally “of full age,” from pərəna-, → full, + āyu- “age,” → aeon.

The slow rotation of the major axis of a planet’s orbit in the same direction as the revolution of the planet itself, due mainly to gravitational interactions with other planets. The perihelion of the planet Mercury advances about 9’.6 per century. The bulk of the advance was accounted by perturbations from other planets. However, a remaining small advance, by 43’’ per century, was eventually explained as an effect predicted by Einstein’s theory of → general relativity. In the case of close binary stars, the advance of pericenter may additionally be caused by mass transfer and the stars’ distorted (elliptical) shapes. Advance of perihelion (or pericenter) is also known as → apsidal motion.

Etymology (EN): Advance, from O.Fr. avancer “move forward,” from V.L. *abantiare, from
L.L. abante “from before,” from ab- “from” + ante “before,” PIE *ant- “front, forehead;” → perihelion.

Etymology (PE): Pišraft “advance,” from piš “forward; in front; before,” Mid.Pers. peš

• raft “going; walk, travel,” from raftan “to go.”

A wave that travels backward in time according to Maxwell’s electromagnetic theory; it arrives before it is transmitted. → Maxwell’s equations have two solutions, the normal solution
describes the ordinary waves, called → retarded waves, traveling forward in time. However, no advanced waves have ever shown up in any experiment. The advanced solutions of Maxwell’s equations are usually simply discarded as “unphysical.”

Etymology (EN): Advanced, adj. from advance, →
advance of perihelion; → wave.

Etymology (PE): Mowj, → wave; pišras “advanced,” from piš “before,” Mid.Pers. peš

• ras “arriving,” from rasidan “to arrive,” Mid.Pers. rasitan, O.Pers./Av. rasa- present stem of ar- “to move, go or come toward,” cf. Skt. ar-, rcchati.

1. Geology: The process of transport of a quantity by the velocity field due to the movement of a fluid. Advection differs from → convection, which describes thermally driven circulation.
   

2. Meteorology: The predominantly horizontal, large-scale motions of the atmosphere. In contrast, convection describes the predominantly vertical, locally induced motions.

Etymology (EN): From L. advecti “act of conveying,” from advectus, past participle of advehere “to carry,” from ad-, “to” + vehere “to carry, bring;” Skt. vah-, vahati “to carry, conduct, guide,” Av. vaz-, vazaiti “to guide, lead”; PIE *wegh- “to go, transport in a vehicle”.

Etymology (PE): Pahnbaz from pahn “flat, wide, → broad,”

• baz “to carry,” → evection.

The first term on the right side in the → induction equation.

See also: → advection; → term.

Adj. from → advection.

A word that serves to qualify a verb, an adjective, another adverb, or an entire sentence. More specifically, adverbs indicate manner, time, place, cause, or degree, and answer questions such as “how,” “when,” “where,” “how much.”

Etymology (EN): From L.L. adverbium “adverb,” literally “that which is added
to a verb,” from → ad- “to” + verbum “word, → verb,” a translation of Gk. epirrhema “adverb,” from → epi- “upon, on” + rhema “verb.”

Etymology (PE): Apivâz, from api-, → epi-, + vâz “word,” → verb.

1. A person, group, or force that opposes or attacks; opponent; enemy; foe.
   

2. A person, group, etc., that is an opponent in a contest; contestant (Dictionary.com).

Etymology (EN): M.E. adversaire; O.Fr. adversaire “opponent, adversary, rival,” from L. adversarius “opposite, hostile, contrary,” from adversus “turned against,” from → ad- + verse, → universe.

Etymology (PE): Hamestâr, from Mid.Pers. hamestâr “adversary,” from Av. hamaēstar-, from ha-, → com-, + maēd- “to bring down, to suppress.”

A round vessel pivoted on a central axis that rotates by the force of internal steam escaping from two diametrically opposed narrow apertures. Aeolipile, first described by Hero of Alexandria (c. 10-70 AD), is an early example of → jet propulsion.

See also: L aeolipila, from Gk aiolipyle, from
Aiolon pyle, fr. aioli-, from Aiolos “god of wind,” +
pyle “gate.”

1. General: An indefinitely long period of time; an age.
   

2. Geology: A unit of geologic time equal to one billion years; US eon.

Etymology (EN): L. aeon, from Gk. aion “age, eternity;” akin to Av. āiiu- “duration, period, lifetime;” Skt. áyu- “life, longevity.”

Etymology (PE): Âyu, from Av. āiiu- “duration, period, lifetime” (Sogd. āy “life, age”), as above.

Of or pertaining to → aerodynamics.

See also: → aerodynamics.

The force exerted by a gaseous fluid upon a body completely immersed in it caused by their relative motion. The components of aerodynamic force are: → lift and → drag.

See also: → aerodynamic; → force.

The science that is concerned with the study of the → motion of → air and other gaseous → fluids and with the → forces acting on bodies moving through such fluids.

See also: Aerodynamics, from Gk. aero-, → air, +
→ dynamics.

A stony meteorite consisting of silicate minerals. This alternative name for a stony meteorite, is now largely obsolete.

Etymology (EN): Aerolite, from Gk. aero-, → air, + Gk. lithos “stone”.

Etymology (PE): Havâsang from Persian havâ “air”; compare with Mid. Pers./Mod. Pers. vây “weather,” from Av. vayah-, vaya- “weather, atmosphere,” from va- “to blow”. Cognate with Skt. va-, Gk. aemi “to blow” + sang, → stone.

A subdivision of meteorology concerned with the total vertical extent of the atmosphere as opposed to the study of the atmosphere near Earth’s surface.

Etymology (EN): Aerology from Gk. aero- “air” + Gk. logia “study of,” from legein “to speak”.

Etymology (PE): Javvšenâsi, from Ar. javv “air, atmosphere” + šenâsi “knowledge, knowing,” from šenâxtan “to know,” from Av./O.Pers. xšnâ “to learn, come to know, know,” compare with Skt. jna “to know,” Gk. gignoskein “to know, think, judge,” L. gnoscere, noscere “to come to know,” PIE *gno- “to know.”

The science and technology concerned with designing, constructing, and operating machines capable of flying in the atmosphere.

Etymology (EN): From aeronautic, from Fr. aéronautique, from aéro-,
from Gk. aer, → air, + nautique “of ships,” from L. nauticus, from Gk. nautikos,
from naus “ship” (cognate with Mod.Pers. nâv “ship;” Av./O.Pers. *nāv-, O.Pers. nāviyā- “fleet;” Skt. nau-, nava- “ship, boat;” Gk. naus, neus, L. navis; PIE *nāu- “ship”).

Etymology (PE): Havânavardi, from havâ, → air, + navardi, verbal noun of navardidan “to travel, walk, pass by and over.”

The study of processes in the the upper atmosphere, especially of regions of ionized gas,
from stratosphere to interplanetary space.

See also: From Gk. aer- “weather” + nomos “arranging, regulating,” related to nemein “to deal out,” → -nomy.

A suspension of fine solid or liquid particles in a gas.

Etymology (EN): Aerosol from aero “air” + sol(ution).

Etymology (PE): Havâlu from havâ “weather” + lu from → luyé, luyeš, → solution.

Relating to the philosophy of → aesthetics.

See also: Adjective from → aesthetics.

1. The branch of philosophy dealing with such notions as the beautiful, the ugly, the sublime, the comic, etc., as applicable to the fine arts, with a view to establishing the meaning and validity of critical judgments concerning works of art, and the principles underlying or justifying such judgments.
   

2. The study of the mind and emotions in relation to the sense of beauty (Dictionary.com).

Etymology (EN): From Ger. Ästhetisch or Fr. esthétique, both from Gk. aisthetikos “sensitive, perceptive,” from aisthanesthai “to perceive, to feel.”

Etymology (PE): Zibâyik, from zibâ “beautiful, elegant, adorned,” → beauty, + -ik, → -ics.

Math.: Of or pertaining to the geometry of → affine transformations. See also: → affine combination, → affine geometry, → affine set, → affinity.

Etymology (EN): From Fr., from L. affinis “bordering on; neighboring; related.”

Etymology (PE): Karvan, literally “bordering on; possessing border (with); related,” from (Bašgard) ker “border, boundary,” (Fin Bandar Abbas) karaq “border, edge;” classical Pers. karân, kenâr “boundary, side, edge;”

• -van suffix of possession, relation.

A linear combination of vectors in which all the vector coefficients add up to one.

See also: → affine; → combination.

The study of the properties which are → invariant under → affine transformations.

See also: → affine; → geometry.

A set → if and only if (iff) for any two points in the → set, the line through them is contained in the set. In other words, for any two points in the set, their → affine combinations are in the set itself.

See also: → affine; → set.

Any → transformation preserving → collinearity.

See also: → affine; → transformation.

1. General: A relationship or resemblance.
   

   2. Math.: The state, quality, or fact of being → affine.

See also: → affine; → -ity.

To state or assert positively; maintain as true.

Etymology (EN): M.E. affermen, affirmen “to decide upon; to state positively,” from O.Fr. afermer “affirm, confirm; strengthen,” from L. affirmare “to make steady, strengthen,” from → ad- “to” + firmare “strengthen, make firm,” from firmus, → firm.

Etymology (PE): Baedešidan, from bar- “upon, to,” + deš, → firm, + infinitive suffix -idan.

The act of affirming or something affirmed.

See also: Verbal noun of → affirm.

A linguistic element added to a word or root to produce a derived or inflected form. General term for → infix, → prefix, → suffix, and → circumfix.

Etymology (EN): From L. affixus “fastened to,” p.p. of affigere
“fasten to, attach,” from af-, variant of → ad- “to”

• figere “fasten.”

Etymology (PE): Vand, variant band “tie, band,” present stem of Mod.-Mid./Pers. bastan/vastan “to bind, shut;” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie;” cf. Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind,” cf. Ger. binden, E. bind.

Describing a lens or optical system with a convergent power of zero, whose object and image points stretch to infinity. Thus,
→ afocal system.

Etymology (EN): Afocal, from → a- + → focal.

Etymology (PE): Akânun, from a- negation prefix, → a- + kânun, → focus. Bikânun, from bi- negation prefix + kânun, → focus.

An optical system with object and image points at infinity.

See also: → afocal; → system.

1. Behind in place or position; following behind.
   

2. Later in time than; in succession to; at the close of (Dictionary.com).

Etymology (EN): M.E., from O.E. æfter “after, next, following in time,” from O.E. of “off,” cf. apo-

• -ter a comparative suffix.

Etymology (PE): Pas “after,” → back-; pasân with place/time suffix -ân.

1. A broad glowing arc of radiance, sometimes seen high in the western sky at → twilight, caused by the → scattering effect of → fine dust particles suspended in the → upper atmosphere.
   

2. Lingering → radiation that remains after a violent → event such as a → gamma-ray burst. The afterglow of any gamma-ray burst is caused by an event different from the original explosion, likely by → shock waves colliding with the ambient medium. Afterglows occur in a large range of → electromagnetic  → wavelengths.

See also: → after; → glow.

The part of day from → noon to → evening.

See also: → after; → noon.

At a later or subsequent time; subsequently.

See also: → after; → -ward.

A → Luminous Blue Variable star in the constellation → Carina; also known as HD 94910. AG Carinae lies about 6 kpc (20,000 → light-years) away and is surrounded by a → nebula. It is also a → spectroscopic variable, with the variability on time-scale of years. During the epochs of minimum in the visual → light curve (m_V ~ 8.1), the star is relatively hot and has a → WN11 spectral type, showing strong He I, H I, and N II → emission lines, weak He II 4686 Å emission, and Si IV 4088-4116 Å absorption. During the maximum epochs of the light curve (m_V ~ 6.0),
the star is cooler, and the spectrum is reminiscent of extreme A-type → hypergiants, with a strong emission of H I, Fe II, and T III lines. The transition between both phases is characterized by the appearance of peculiar features in the spectrum, such as absorption-line splitting, strong → electron-scattering wings in He I and Fe II lines, and apparent → inverse P Cygni profiles in He I lines.

The presence of a massive → bipolar nebula around AG Car testifies to a recent (t< 10⁴ years) phase of high → mass loss. The morphology and kinematics of the nebula suggest a → dynamical age of 8.5 × 10³ years and a high mass of → ionized nebular material (~ 4.2 Msun), which is likely composed of → ejecta from the central star. The nebular abundances show evidence of moderate nitrogen → enrichment. Properties of the → circumstellar nebula, studied in the → mid-infrared and → far-infrared, reveal an incredibly high dust mass of ~ 0.25 Msun, → dust temperature between 76 and 99 K, and the presence of large → dust grains of ~ 1 μm, as deduced from far-→ infrared excess. Assuming a normal → gas-to-dust ratio of 100, the total nebular mass of AG Car could be as high as ~ 30 Msun which would be of the order of, or even higher than, the mass of the → Homunculus nebula around → Eta Carinae. At the time when the AG Car nebula was ejected, the → interstellar bubble around the central star likely contained a negligible amount of material compared to the total mass of the nebula, implying that most of the nebular mass was ejected by the central star (Groh et al., 2009, ApJ 698, 1698).

See also: AG, as used in the → variable star designation system; → Carina.

Once more; a second time; in return.

Etymology (EN): M.E., from O.E. ongean “toward, opposite, against,” from on “on” + -gegn “against, toward.”

Etymology (PE): Dobâré, from do, → two, + bâr “time, fold,” → twice.

In opposition to; contrary to; adverse or hostile to.

Etymology (EN): M.E. agens, ageynes, from ageyn, ongean “again,” from on + gegn “against, toward,” → gegenschein.

Etymology (PE): Patâr from Mid.Pers. paitiyârak, “every thing that counteracts and checks the good powers: adversaries of the evil powers;” Av. paiti-ār- “to advance against,” paiti-ārəna- “adversary,” from paiti- “against,” → counter-, + ar- “to move,” → access; cf.
Mod.Pers. patyâré “affliction, misery, misfortune; anything ugly, terrific;” Mid.Pers. padirag, patirak “against, counter-,” Sogd. patrêt “against, opposite, toward.”

In evolutionary models of → low-mass and → intermediate-mass stars, the occurrence of a → helium shell flash just at the moment when the star is leaving the → asymptotic giant branch phase.

See also: → AGB; → final; → thermal; → pulse.

1. The length of time that a celestial body or an evolutionary stage of it has existed, i.e. the age of a massive star, the age of a galaxy, a stellar cluster, and so on.
   
2. a: A period of time in the history of the Universe marked by a distinctive characteristic. → cosmic dark age. b: A division of geologic time in the history of the Earth, usually shorter than an epoch.

Etymology (EN): From O.Fr. aage, from Vulgar L. *ætaticum, from L. ætatem “period of life,” from ævum “lifetime, eternity, age;” cf. Mid.Pers. awâm, âwâm “time, season,” Av. âyav- “duration, period, time of life,” Skt. âyuh- “life, health”. All
from PIE *aiw-, *ayu- “vital force, life, long life, eternity.”