An Etymological Dictionary of Astronomy and Astrophysics

The seventeenth of Saturn’s known satellites and the third largest. It is 1436 km across and revolves around Saturn at a mean distance of 3.5 million km with a period of 79 days. The most unique and perhaps most remarkable feature on Iapetus is a topographic ridge 13 km higher than the surrounding terrain, as discovered in the images obtained with Cassini spacecraft. The ridge extends at least 1300 km almost exactly parallel with Iapetus’s equator.

Etymology (EN): Iapetus was one of the Titan gods, sons of Ouranos (Heaven) and Gaia (Earth). Discovered 25 October 1671 by Jean-Dominique Cassini.

A small → dwarf irregular galaxy belonging to the → Local Group, which is located in the constellation → Cassiopeia.
IC 10 is about 2.3 million light-years distant and about 5,000 light-years across. With an absolute B magnitude of -16.5, IC 10 has an integrated luminosity that is comparable to that of the → Small Magellanic Cloud, although it is considerably smaller. Its → metallicity is roughly a factor of 2 higher than that of the SMC and in the same proportion smaller than → LMC’s. Compared to other Local Group galaxies, IC 10 has a large population of newly formed stars that are massive and intrinsically very bright, especially → Wolf-Rayet stars. Its W-R star density is larger than those of the LMC and SMC. Moreover, the relative number of known → WC to → WN type Wolf-Rayet stars is unusually high, which is unexpected for IC 10’s metallicity. IC 10 is also known to be unusual in having → H I gas that extends about 7 times the optical dimensions of the galaxy. IC 10 is the nearest example of a → starburst galaxy. IC 10 was discovered by the American astronomer Lewis A. Swift (1820-1913) in 1887.

See also: Number 10 in the → Index Catalogue.

A → dwarf irregular galaxy (also called Caldwell 51) located in the constellation → Cetus. IC 1613 is a member of the → Local Group of galaxies and has a → morphological classification of IBm (Irregular Barred, → Magellanic type galaxy). IC 1613 has a very → low surface brightness and a very low → dust content. It lies just over 2.38 &plusmn 0.07 million → light-years
away from us. IC 1613’s distance is known to a remarkably high precision, partly due to the unusually low levels of dust lying both within the galaxy and along the line of sight from the → Milky Way. Moreover, the galaxy hosts a number of → Cepheid variables and → RR Lyrae variables which can be used to precisely determine distances. It was discovered by Max Wolf in 1906.

See also: IC, short for → Index Catalogue.

A bright → emission nebula running north to south near → Alnitak, the eastern star of → Orion’s Belt. It is the bright background nebula against which the famous dark region called the → Horsehead Nebula stands out.

See also: IC, → Index Catalogue.

1. The solid form of water; it is found in the atmosphere as snow crystals, hail, ice pellets, etc., and on the Earth’s surface in forms such as frost, rime, glaze, glacier ice, etc.
   
2. The solid, frozen form of gases in cold astronomical objects.

Etymology (EN): Ice, from O.E. is “ice,” from P.Gmc. *isa-; cf. O.N. iss, O.Fris. is, Du. ijs, Ger. Eis. Cognate with Pers. yax, as below.

Etymology (PE): Yax, from Av. aexa- “ice, frost,” isav-, isu- “icy, chilly;” cf. Sarikoli (Pamir dialect) īš “cold;”
P.Gmc. *isa-, as above.

Meteo.: The process by which a layer of ice builds up on solid objects that are exposed to freezing precipitation or to supercooled fog or cloud droplets.

See also: → ice; → accretion.

A major interval of geologic time during which extensive ice sheets (continental → glaciers) formed over many parts of the world.

There have been at least five significant ice ages in Earth’s history, with approximately a dozen epochs of glacial expansion occurring in the past 1 million years. The last one ran from about 75,000 to 15,000 years ago.

See also: → ice; → age.

A tiny particle of ice within which → water molecules are all lined up in a precise → crystalline structure. Ice crystals come in various shapes including needles, → dendrites, hexagonal columns, and → platelet. If the temperature decreases the water molecules can crystallize, arranging themselves around the suspended impurities such as dust particles. See also → snow crystal.

See also: → ice; → crystal.

A member of the lesser mass group of → gas giants. Ice giants contain a higher quantity of materials that form ices at low temperatures, such as → water, → methane, and → ammonia. There are two ice giants in the Solar System, → Uranus and → Neptune.

See also: → ice; → giant.

A building for storing ice throughout the year, mainly used prior to the invention of the refrigerator.

Etymology (EN): → ice; → house.

Etymology (PE): Yaxcâl, literally “a pit of ice,” but “any place or vessel in which ice is kept,”
from yax, → ice, + câl “pit, → hole.”

The temperature at which a mixture of air-saturated pure water and pure ice may exist in equilibrium at a pressure of one standard atmosphere.

See also: → ice; → point.

A large mass of floating or stranded ice that has broken away from a glacier; usually more than 5 m above sea level.

Etymology (EN): Iceberg, half Anglicization, half adoption of
Du. ijsberg “ice mountain,” from ijs, → ice,

• berg “mountain” (cf. Ger. Eisberg, Sw. isberg), from P.Gmc. *bergaz; cf. M.H.G. berc, O.H.G. berg “mountain;” cognate with Pers. boland, borz, berg, as below; PIE base *bheregh- “high, elevated.”

Etymology (PE): Yaxberg, from yax, → ice,

• berg “mountain, hill,” in Laki dialect, related to Pers. boland “high,” variants bâlâ “up, above, high, elevated, height,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
  Laki dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”
  

1. An image; a representation.
   

   2. A representation or picture of a sacred or sanctified Christian personage, traditionally used and venerated in the Eastern Church.
      

   3. A person or thing regarded as a symbol of a belief, nation, community, or cultural movement.
      

   4. Computers: A pictorial representation of a facility available on a computer system, that enables the facility to be activated by means of a screen cursor rather than by a textual instruction (TheFreeDictionary.com).

Etymology (EN): From Gk. eikon “likeness, image, portrait; a semblance;” in philosophy, “an image in the mind,” related to eikenai “be like, look like,” from PIE *weik- “to be like.”

Etymology (PE): Zodiš, variant of Mid.Pers. uzdês “icon; image; idol,” from uz-, → ex-, + dês, Av. daēs- “to show;” cognate with Gk. deiknumai “to show;” L. dicere “to utter, say, proclaim;” N.H.G. zeigen “to say;” O.E. têon “to annoince;” PIE
*deik- “to show, point out; announce.”

Made of, covered with, or containing → ice.

See also: → ice + an English suffix of adjectives denoting “characterized by or inclined to.”

A thought, conception, or notion existing in the mind as a result of mental understanding, awareness, or activity. See also → thought,
→ concept.

Etymology (EN): Idea, from L. idea “idea,” pre-Platonic Gk. idea “form, semblance, nature, fashion,” in Plato “a timeless, universal archetype of existents; ideal prototype,”
literally “look, form,” from idein “to see,” from PIE *wid-es-ya-, suffixed form of base *weid- “to know, to see;” cf. Pers. bin- “to see” (present stem of didan); Mid.Pers. wyn-;
O.Pers. vain- “to see;” Av. vaēn- “to see;”
Skt. veda “I know.”

Etymology (PE): Miné “idea,” related to Pers. maneš “disposition, temperament, greatness of soul,” minu “heaven, paradise,” also equivalent to Ger. Geist in recent philosophical translations, došman “enemy,” pašimân “penitent, regretful,” pežmân “sad, mournful,” šâdmân “joyful, cheerful, pleased,” ârmân “desire; → ideal;” dialectal (Šuštar) mana “(he) thinks, imagines,” (Tarq-e Natanz) môna “to imagine, suppose;” Mid.Pers. mênidan “to think, consider,” mên “thought, idea,” mênišn “thought, thinking, mind, disposition,” mênitâr “thinker,” mênôg “spiritual, immaterial, heavenly,” from Av. man- “to think,” mainyeite “he thinks,” manah- “mind, thinking, thought; purpose, intention,” mainyu- “mind, mentality, mental force, inspiration,” traditionally translated as “spirit,” Angra Mainyu “hostile mentality” (Mod.Pers. Ahriman); O.Pers. maniyaiy “I think,” Ardumaniš- (proper noun) “upright-minded,” Haxāmaniš- (proper noun, Hellenized Achaemenes, founder of the Achaemenian dynasty) “having the mind of a friend;” cf. Sogdian mân “mind;” Skt. man- “to think,” mánye “I think,” manyate “he thinks,” mánas- “intelligence, understanding, conscience;” Gk. mainomai “to be angry,” mania “madness,” mantis “one who divines, prophet;” L. mens “mind, understanding, reason,” memini “I remember,” mentio “remembrance;” Lith. mintis “thought, idea;” Goth. muns “thought,” munan “to think;” Ger. Minne “love,”
originally “loving memory;” O.E. gemynd “memory, thinking, intention;” PIE base
*men- “to think, mind; spiritual activity.”

1. (n.) A standard of perfection, beauty, or excellence.
   Math.: A subset of a ring that is closed under addition and multiplication by any element of the ring.
   
2. (adj.) Existing only in the imagination; not real or actual.
   Conforming exactly to an ideal, law, or standard; perfect. → ideal gas.

Etymology (EN): M.E. ydeall, from L.L. idealis “existing in idea,” from L. → idea.

Etymology (PE): Ârmân “ideal” in Mod.Pers., traditionally “desire; hope; grief,” variants armân, urmân, prefixed from mân, “thought, mind,” → idea. The first element may be related to Av. armaē- “in peace, still; quietly;” PIE base *er[ə]- “to be still” (cf. Skt. īrmā (adv.) “quiet, still, being in the same place;” Gk. erôé “calm, peace;” O.H.G. rouwa “rest”), as in Av. armaē.šad- “sitting quietly,” armaē.štā- “standing still, stagnant.” Therefore, Pers. ârmân may be related to Av. *armaē.manah- (PIE *ermen-) “thought in peace, quiet mind.”
Mineyi, minevâr, adj. from miné, → idea.

An fluid which has no internal friction or → viscosity, and no → resistivity as well if the fluid is a plasma.

See also: → ideal; → fluid.

Theoretical gas assumed to consist of perfectly elastic molecules of negligible volume and mutual attraction force. Also called → perfect gas.

See also: → ideal; → gas.

An → equation of state that relates pressure (P), temperature (T), and volume (V) of an ideal or → perfect gas: PV = nRT, where n is the number of → moles of gas present and R is the → universal gas constant. Equivalently: PV = NkT, where N is the number of atoms of gas present and k is → Boltzmann’s constant.

See also: → ideal; → gas; → law.

Magnetohydrodynamics of a → plasma with very large (infinite) → conductivity. In this condition, → Ohm’s law reduces to E = -v × B, where E represents → electric field, B → magnetic field, and v the → fluid velocity.

Ideal MHD is the simplest model to describe the dynamics of plasmas immersed in a magnetic field. It is concerned with
→ one-fluid magnetohydrodynamics and neglects → resistivity. This theory treats the plasma composed of many charged particles with locally neutral charge as a continuous single → fluid. Ideal MHD does not provide information on the velocity distribution and neglects the physics relating to wave-particle interactions, as does the two-fluid theory as well. It does have the advantage that the macroscopic dynamics of the → magnetized plasma can be analyzed in realistic three-dimensional geometries (K. Nishikawa & M. Wakatani, 2000, Plasma Physics, Springer). See also → non-ideal magnetohydrodynamics.

See also: → ideal; → magnetohydrodynamics.

Any one of a variety of systems of philosophical thought, which would make the ultimate reality of the Universe expressible or intangible only in terms of idea in mind, rather than in terms of matter in space.

See also: From → ideal, see also → idea,

• → -ism.

Similar or alike in every way; being the very same; agreeing exactly.

Etymology (EN): From M.L. identicus “the same,” from L.L. identitas “identity,” from idem “the same,” from id “it, that one” + demonstrative suffix -dem.

Etymology (PE): Yeksân “the same, similar,” from yek, → one, + -sân “manner, semblance” (variant sun, Mid.Pers. sân “manner, kind,” Sogdian šôné “career”).
Hamidân, from Mid.Pers. hamêtôn “identical.”

1. An act or instance of identifying; the state of being identified.
   

2. Psychology: The process of associating the self closely with other persons and assuming their characteristics or views unconsciously or partially so.
   

3. In psychological theories, a phase of recognition, the way in which the child’s personality is formed by modeling it on a chosen parent.

Etymology (EN): Verbal noun of → identify.

Etymology (PE): Idâneš, verbal noun of idânidan→ identify.

Recognizing the lines in the spectrum of a star, nebula, galaxy, etc. using a calibration template.

See also: → identification; → line.

Computers:

1. One or more characters used to identify, name, or characterize the nature, properties, or contents of a set of data elements.
   
2. A string of bits or characters that names an entity, such as a program, device, or system, in order that other entities can call that entity.
   
3. In programming languages, a lexical unit that names a language object, such as a variable, array, record, label, or procedure.

See also: Agent noun of → identify.

To recognize or establish as being a particular person or thing; e.g.
→ identification of lines.

Etymology (EN): Identify, from Fr. identifier, from identité, → identity.

Etymology (PE): Idânidan, infinitive from idâni, → identity.

1. Math.: An equation that is valid for all values of its variables. A mathematical relationship equating one quantity to another.
   

2. Logic: An assertion that two terms refer to the same thing.
   

3. Psychology: The character of persisting unchanged. The feeling that one knows who one really is.

Etymology (EN): From M.Fr. identité, from L.L. identitas
“sameness,” from ident-, combining form of L. idem “the same; at the same time; also; moreover,” from id “it, that one” + demonstrative suffix -dem + -itas a suffix used to form abstract nouns expressing state or condition.

Etymology (PE): Idâni, from iduni, from Mid.Pers. êdônih “being this, being that, being so, the manner of being,” from êdôn “thus, so,” Mod.Pers. idun “this, in this manner, now;” O.Pers. aita- demonstrative pronoun “this;” Av. aēta- “this; this here; this now,”
aētaδ- (adv.) “here, there; then, thus; thereupon;” cf. Skt. etad “this,” iti “thus, in this manner;” akin to L. idem, as above.

A basic rule in → group theory stating that there exists a unit group element e, called the identity, such that
for any element a of the group a * e = e * a = a.

See also: → identity; → axiom.

In a mathematical system, an element which leaves unchanged any other element on which it operates. Thus 0 is the identity element for addition: a + 0 = a. And 1 is the identity element for multiplication: a . 1 = a.

See also: → identity; → element.

Math.: Any function f for which f(x) = x for all x in the domain of definition.

See also: → identity; → function.

In linear algebra, the simplest nontrivial diagonal matrix, an n-by-n square matrix with ones on the main diagonal and zeros elsewhere.

See also: → identity; → matrix.

An operator which takes a real number to the same real number.

See also: → identity; → operator.

(Conjunction) in case that; granting or supposing that; on condition that. → if and only if.

Etymology (EN): M.E., from O.E. gif; akin to O.H.G. ibu “if.”

Etymology (PE): Agar “if,” from Mid.Pers. agar, hakar “if;” O.Pers. ha-karam “once;” from ha- “one,” variant ham-, → com- (cf. Av. ha-; Skt. sa-; Gk. ha-; L. sem-; PIE *sem- “one”), + karam- “a time,” maybe from kar- “to do; doing,” → work; cf.
Av. hakərət “once;” Skt. sakrt “once;” Gk. hapaks “once;” L. semel “once.”

Logic, Math.: An → expression indicating that two → statements so connected are → necessary and sufficient conditions for one another. The corresponding logical symbols usually used are: ↔, ⇔, ≡, and iff.

See also: → if; → and; → only.

A → rock formed by the solidification of molten material that originated within the Earth (as → magma or → lava).

Etymology (EN): Igneous, from L. igneus “of fire, fiery,” from ignis “fire,” from PIE *egni- (cf. Skt. agni- “fire, sacrificial fire,” O.C.S. ogni, Lith. ugnis “fire”); → rock.

Etymology (PE): Sang “stone, rock,” → stone; âzarin “fiery,” from âzar, variants âtaš, taš, from Mid.Pers. âtaxš, âtur “fire;” Av. ātar-, āθr- “fire,” singular nominative ātarš-; O.Pers. ātar- “fire;” Av. āθaurvan- “fire priest;” Skt. átharvan- “fire priest;” cf. L. ater “black” (“blackened by fire”); Arm. airem “burns;” Serb. vatra “fire;” PIE base *āter- “fire” + -in adj. suffix.

1. To set on fire.
   
2. To take fire, to begin to burn.

Etymology (EN): From L. ignitus, p.p. of ignire “to set fire,” from ignis “fire, flame, light.”

Etymology (PE): Girândan transitive form of gereftan

“to take, catch” (Mid.Pers. griftan, Av./O.Pers. grab- “to take, seize,” cf.
Skt. grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving,” M.L.G. grabben “to grab,” from P.Gmc. *grab, E. grab “to take or grasp suddenly;” PIE base *ghrebh- “to seize”).

1. The act or fact of igniting; state of being ignited.
   
2. The process of setting something on fire.

See also: Verbal noun of → ignite.

The minimum temperature to which a fuel must be heated in order to initiate self sustained combustion independent of another heat source.

See also: → ignition; → temperature.

A → generalized coordinate that does not appear explicitly in the → Lagrangian function. Also called → cyclic coordinate. Ignorable coordinates do not participate in the → Legendre transformation, and
are also absent from the → Hamiltonian function. The generalized → momentum associated with an ignorable coordinate is a → constant of the motion.

See also: → ignore; → coordinate.

To set aside, e.g. → ignorable coordinate.

Etymology (EN): From Fr. ignorer, from L. ignorare “not to know, disregard,” from ignarus “not knowing, unaware,” from in- “not,”
→ un-, + gnarus “aware, acquainted with,” related to gnoscere “to → know”.

Etymology (PE): Nâdidé engâštan literally “supposed not seen,” from nâdidé “not seen” (from nâ- “not,” → un- + didé “seen,” p.p. of didan “to see,” → sight) + engâštan “to suppose” (→ hypothesis).

Optics: Intensity of → illumination.
The amount of visible light incident per unit area of surface per second; measured in lux (lumens per unit square meter). → irradiance.

See also: Rowšanâyi, → illumination; tanuyi→ intensity.

General: An act or instance of illuminating. The fact or condition of
being illuminated.

Etymology (EN): Illumination, from O.Fr. illumination, from L. illuminationem (nom. illuminatio), from illuminare “to throw into light,” from → in- “in” + lumen “light,” cognate with Pers.
rowšan, as below.

Etymology (PE): Rowšanâyi, noun of quality and state from rowšan “bright, clear,” related to ruz “day,” foruq “light,” and afruxtan “to light, kindle,” rowzané “window, aperture;” Mid.Pers. rôšn “light; bright, luminous,” rôc “day,” rocânak “window;” O.Pers. raucah-, Av. raocana- “bright, shining, radiant,”
raocah- “light, luminous; daylight;” akin to Skt. rocaná- “bright, shining,” roka- “brightness, light;” Gk. leukos “white, clear;” L. lux “light,” also lumen “light, window,” luna “Moon;” E. light; Ger. Licht; Fr. lumière; PIE base *leuk- “light, brightness.”

1. A false sense impression of something actually present.
   

2. Something that produces a false or misleading impression of reality.
   → optical illusion.
   

3. An instance of being deceived.

Etymology (EN): From O.Fr. illusion “a mocking,” from L. illusionem (nom. illusio) “a mocking, irony,” from illudere “to mock at,” literally “to play with,” from in- “at” + ludere “to play,” from ludus “a game, play;” PIE base *leid- “to play, jest.”

Etymology (PE): Safmân, literally “wrong thought, ~ idea,” from saf “wrong” (Tâleši), → false, + mân “thought,” → idea.

1. In an optical system, the point to which light rays are converged (real image) or from which they appear to diverge (virtual image) after reflection or refraction.
   Math.: A number, point, or unique element of a space that corresponds to some other number, point, or unit element.
   

2. To make an image of; to picture or represent in the mind.

Etymology (EN): From O.Fr. image, from L. imaginem (nom. imago) “copy, picture, likeness " from stem of imitari “to copy, imitate,” from im-, stem of imitare “to copy, imitate” + -ago noun suffix.

Etymology (PE): 1) Vine, from (Sorani) Kurd. vina or vena “image, picture; similar,” from vin/ven, variant of bin, present stem of dîtin, Pers. didan “to see, to look;” cf. Taleshi vinde “to see,” Tati vindiyan “to see,” Persian bin-, didan “→ see.” The relation between “image” and “see” is probably from the fact that the image of a subject can be seen without its presence.
Tasvir, loan from Ar.

2. Infinitive with gereftan “take,” → take, and dâštan “to have, to hold,” → property.

The process of adding several usually low-exposure images to create an image having a significantly higher signal/noise ratio.

See also: → image; → co-; → add.

A technique using a mathematical model of the blurring process to recover the original, sharp image. See also → blurred image.

See also: → image; → deburring.

The meeting of light rays emanating from an object after crossing an optical system.

See also: → image; → formation.

Device that produces an observable image that is brighter at output than the image at input.

See also: → image; → intensifier.

The plane in which is formed an image produced by an → optical system.

See also: → image; → plane.

One of the points constituting an extended image formed in an → optical system.

See also: → image; → point.

The use of techniques to produce, extract, identify, and display images for evaluation, interpretation, and further interaction with the data.

See also: → image; → processing.

1. The degree of visibility of relevant → information
   in an image.
   

2. The → full-width at half-maximum of a long exposure stellar image obtained by an instrument mounted on a telescope observing in the presence of → seeing.

See also: → image; → quality.

The separation between two detached but adjacent points in an image.

See also: → image; → resolution.

The process by virtue of which the original image can be created by removing the blurring and the noise that occur during image formation.

See also: → image; → restoration.

The quantity that relates the length on the image to the angular or physical separations on the sky.

See also: → image; → scale.

In an → optical system, the space defined by the totality of → image points. The corresponding points in image space and → object space are called → conjugate points.

See also: → image; → space.

A device for amplifying faint images by electronic means. Electrons, released when the incident radiation forms a pattern on a photocathode, are accelerated by a magnetic field so that they form a second, brighter image when they strike a phosphor screen.

See also: → image; → tube.

An electronic device which records images.

See also: → image; → -er.

1. The formation of mental images, figures, or likenesses of things, or of such images collectively.
   

   2. Pictorial images, as in works of art (Dictionary.com).

See also: Noun from → imager.

Capable of being imagined or conceived.

See also: → imagine; → -able.

1. Existing only in the imagination.
   

   2. Math.: → imaginary number.

See also: → imagine; → -ary.

A number that is or can be expressed as the square root of a negative number; thus √ -1 is an imaginary number, denoted by i; i² = - 1.

See also: → imaginary; → number.

1. The faculty of imagining, or of forming mental images or concepts of what is not actually present to the senses.
   

   2. The action or process of forming such images or concepts (Dictionary.com).

See also: → imagine; → -tion.

Characterized by or bearing evidence of imagination (Dictionary.com).

See also: → imagination; → -ive.

1. To form a mental image of (something not actually present to the senses).
   

   2. To think, believe, or fancy.
      

   3. To assume; suppose.
      

   4. To conjecture; guess (Dictionary.com).

Etymology (EN): M.E. imaginen, from M.Fr. imaginer “sculpt, carve, paint; decorate,” from L. imaginari “to form a mental picture, picture to oneself, imagine,” from imago “an image, a likeness,” from stem of imitari “to copy, imitate” (from PIE root *aim- “to copy”).

Etymology (PE): Vinârdan, from vin, short for vine, → image, + ârdan, short for âvardan “to bring; to cause, produce” (Mid.Pers. âwurtan, âvaritan; Av. ābar- “to bring; to possess,” from prefix ā- + Av./O.Pers. bar- “to bear, carry,” bareθre “to bear (infinitive),” bareθri “a female that bears (children), a mother;” Mod.Pers. bordan “to carry;” Skt. bharati “he carries;” Gk. pherein; L. fero “to carry”).

The visual representation of an astronomical body using a two-dimensional detector and computerized techniques.

Etymology (EN): From → image + → -ing.

Etymology (PE): Tasvirgari, from tasvirgar agent noun from tasvir→ image + -gar
from kar-, 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 method used to detect very brief flashes of → Cherenkov radiation generated by the → cascade shower of → relativistic charged particles produced when a very high-energy → gamma ray (in the range 50 GeV to 50 TeV) strikes the atmosphere at a typical altitude of 10 km. Owing to this technique, it possible to discriminate cosmic gamma rays from the cosmic ray background and to determine their energy and source direction. More specifically, the incoming gamma-ray photon undergoes → pair production in the vicinity of the nucleus of an atmospheric molecule. The electron-positron pairs produced are of extremely high energy and immediately radiate in a → bremsstrahlung process. This radiation is itself extremely energetic, with many of the photons undergoing further pair production. A cascade of charged particles ensues which, due to its extreme energy, produces a flash of Cherenkov radiation lasting between 5 and 20 nano-seconds. The total area on the ground illuminated by this flash corresponds to many hundreds of square meters, which is why the effective area of IACT telescopes should be large.

See also: → imaging; → atmospheric; → Cherenkov; → technique.

A detector with two-dimensional capability, such as a CCD.

See also: → image; → → scale.

1. Occurring or accomplished without delay; following or preceding without a lapse of time. → instant.
   

2. Having no object or space → intervening; without intervening medium or agent; direct (Dictionary.com).

Etymology (EN): M.E. immediat, from O.Fr. immediat, from L.L. immediatus “without anything between,” from → in- “not, opposite of,” + mediatus, p.p. of mediare “to be in the middle,” from L. medius “middle,” → medium.

Etymology (PE): Âni, from ân loan from Ar. ân “time, a short time.”
Amadim, literally “with nothing interposed,” from the negation prefix → a- + madium, → medium.

To plunge into or place under a liquid; dip; sink (Dictionary.com).

See also: → in- + merge, → submerge.

The disappearance of a star, planet, moon, or other body at the beginning of an → occultation or → eclipse. Also called → ingress.

See also: → immerge.

To come to a country of which one is not a native, usually for permanent residence.

See also: → in-; → migrate.

A collision between two bodies. In the case of solar system objects,
when one is much smaller than the other (like a meteoroid colliding with the Earth), a crater may be produced on the larger body.

Etymology (EN): From L. impactus, p.p. of impingere “to drive into, strike against,” from → in- “in” + pangere “to fix, fasten.”

Etymology (PE): Barxord, verbal noun of barxordan “to collide, clash, dash against each other,” from bar- “on, upon, up” (Mid.Pers. abar; O.Pers. upariy “above; over, upon, according to;” Av. upairi “above, over,” upairi.zəma- “located above the earth;” cf. Gk. hyper- “over, above;” L. super-; O.H.G. ubir “over;” PIE base *uper “over”)

• xordan “to hit, strike,” originally “to eat, drink, devour,” and by extension “to destroy,” from Mid.Pers. xvardan “to eat, enjoy (food),” Av. x^var- “to consume, eat;” Laki dialect hovârden “to eat;” Proto-Iranian *huar- “to consume, eat.”

A depression produced by the collision of a meteorite, asteroid, or comet with the surface of a planet or a satellite. Impact craters are the most characteristic surface features of solar system rigid bodies. They range in size up to hundreds or thousands of kilometers (where the impacts create giant basins as on the Moon, Mars, and Mercury).

See also: → impact; → crater.

An → atmospheric escape mechanism that occurs where atmospheric gases are expelled en masse as a result of large body impacts, such as the cumulative effect of asteroids hits (see, e.g., Catling, D. C. and Kasting, J. F., 2017, Escape of Atmospheres to Space, pp. 129-167. Cambridge University Press).

See also: → impact; → erosion.

A collision between two celestial objects, specially solar system bodies, with considerable consequences. Impact events involve release of large amounts of energy. Some examples are the 1908 Siberian → Tunguska event by a → comet, the → Barringer Crater, and the collision of an → asteroid with Earth 65 million years ago, which is thought to have led to the extinction of the dinosaurs and other species of the → Cretaceous-Paleogene period.

See also: → impact; → event.

The danger of collision with Earth posed by solar system small bodies that pass near our planet. These objects include → near-Earth asteroids and nuclei of → comets. See also: → near-Earth object, → impact crater, → Torino scale, → Palermo scale, → Space Situational Awareness.

See also: → impact; → hazard.

The loss of orbital electrons by an atom of a crystal lattice which has undergone a high-energy collision.

See also: → impact; → ionization.

1. A measure of the distance by which a collision fails being frontal.
   

2. In → gravitational lensing, the distance of closest approach of the light path to the → lensing object.
   

3. In → rainbows, the displacement of the → incident from an axis that passes through the center of the water droplet.

See also: → impact; → parameter

The enormous drop in temperature and the related effects of the shrouding of Earth with soot and dust particles after the planet is struck by a sizable comet or asteroid. Such a phenomenon is believed to have killed off the dinosaurs 65 million years ago.

See also: → impact; → winter.

A general term used for all rocks affected by, or produced by, the → shock waves and other processes generated by hypervelocity → meteorite → impact events. Impactites occur in and around the → impact crater, typically as individual bodies composed of mixtures of melt and rock fragments, often with traces of meteoritic material.

See also: → impact; → -ite.

A natural impacting body, such as a comet, asteroid, or planet. It can also be a space probe designed to collide with an astronomical body in the solar system.

Etymology (EN): Impactor, from → impact + -or a suffix forming agent nouns.

Etymology (PE): Barxordgar, from barxord, → impact,

• -gar agent suffix, from kar-, 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”).

1. To give, convey, or grant from or as if from a store.
   

2. To communicate the knowledge of; disclose.

Etymology (EN): M.E., from O.Fr. impartir, from L.L. impartire “to share in, divide with another, communicate,” from assimilated form of → in- “into, in” + partire “to divide, → part.”

Etymology (PE): Darpârdan, from dar-, → in-, + pâr, → part, Mid.Pers. pârag “part, portion; gift, offering;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assign;” + -dan infinitive suffix.

General: The ratio of a quantity with the nature of a force to a related quantity with the nature of a current.
Electricity: The apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current

Etymology (EN): From impede, from L. impedire “to entangle,” literally “to shackle the feet,” from in- “in” + pes (gen. pedis) “foot” + -ance a suffix used to form nouns either from adjectives in -ant or from verbs.

Etymology (PE): Pâgiri, verbal noun of pâgir “impedder, impeded; hinderer, hindered,” from pâ “foot, step” (from
Mid.Pers. pâd, pây; Av. pad- “foot;” cf. Skt. pat; Gk. pos, gen. podos; L. pes, gen. pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + gir present stem of gereftan
“to take, seize” (Mid.Pers. griftan, Av./O.Pers. grab- “to take, seize,” cf.
Skt. grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving,” M.L.G. grabben “to grab,” from P.Gmc. *grab, E. grab “to take or grasp suddenly;” PIE base *ghrebh- “to seize”).

Latin verb meaning “to command, rule, reign.”

Etymology (EN): L. imperare “to command, give orders, exercise authority,” from → in- “into, in” + parare “to make ready, supply, order,” related to parire “produce, give birth to,” from PIE root *pere- “to produce, procure;” cf. Skt. prthukah “child, calf, young of an animal;” Gk. poris “calf, bull;” Czech spratek “brat, premature calf;” Lith. periu, pereti “to brood;” O.H.G. farro, Ger. Farre, Du. varre “bull,” O.E. fearr “bull;” see below for possible Iranian cognates.

Etymology (PE): Parmâtidan, from BMP plm’(d)y “to command, order,” Sogd. framat- “to command,” variants of farmudan, farmâyidan “to command, to order,” ultimately from prefixed Proto-Ir. *fra-maH-, from *maH- “to measure,” → experiment.

1a) A command or order.

1b) → imperative case.

2a) Absolutely necessary or required; extremely important.

2b) Of the nature of or expressing a command; commanding.

Etymology (EN): From L.L. imperativus “pertaining to a command,” from imperat-, p.p. stem of → imperare “to command.”

Etymology (PE): Parmât, noun from present stem of parmâtidan, → imperare; parmâtii, adj. from parmât.

The grammatical mood of a verb that expresses a command or a request, as in close the door!.

See also: → imperative; → case.

In an imperative manner.

See also: Adverb from → imperative; → -ly.

Not perfect; faulty or incomplete.

See also: → in-; → perfect.

A fault, flaw, or undesirable feature; the state or condition of being imperfect.

See also: → in-; → perfection.

Of, relating to, or suggestive of an empire or a sovereign, especially an emperor or empress (TheFreeDictionary).

See also: Adjective of → empire.

1. The extension of a nation’s authority by territorial acquisition or by the establishment of economic and political dominance over other nations.
   

   2. A political doctrine or system promoting such extension of authority (TheFreeDictionary.com).

See also: → imperial; → -ism.

1. General: Something implied; the act of implying.
   

2. A statement that follows from other given statements. For ex., an implication of x = -1 is that x² = 1.
   

3. A proposition found from two given propositions by connecting them in the form “If …, then …”. The first statement is the antecedent (or hypothesis) and the second the consequent (or conclusion).

Etymology (EN): From L. implicationem “interweaving, entanglement,” from implicatus, p.p. of implicare “involve, entangle, connect closely,” from → in- “in” + plicare “to fold,” → explain.

Etymology (PE): Dartâheš, verbal noun of dartâhidan “to imply,” from dar-, → in- + tâhidan “to fold, ply,” → explain.

1. General: Implied, rather than expressly stated.
   

2. Math.: → implicit function.

Etymology (EN): From L. implicitus, variant of implicatus, p.p. of implicare, → implication.

Etymology (PE): Dartâhi, from dartâh present stem of dartâhidan,
→ implication, + -y adj. suffix.

A function which contains two or more variables that are not independent of each other. An implicit function of x and y is one of the form f(x,y) = 0, e.g., 4x + y² - 9 = 0. See also → explicit function.

See also: → implicit; → function.

To collapse, or cause to → collapse, inward → violently. Opposite of → explode.

Etymology (EN): Implode, from → in- + (ex)plode, from
L. plaudere “to clap, beat,” of uncertain origin.

Etymology (PE): Forukaftidan, from foru- “down, downward; below; beneath” (Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain”) + kaftidan “to burst; to split,” variants kaftan, kâftan “to split; to dig,” (Parthian Mid.Pers. q’f- “to split;” Sogdian kβ “to spli;” Chorasmian kf- “to split, be split;” Proto-Iranian *kap-, *kaf- “to split”).

A violent inward collapse; the act of imploding; opposed to → explosion.

See also: Verbal noun of → implode.

1. To involve or indicate by inference, association, or necessary consequence rather than by direct statement .
   

2. To contain potentially.
   

3. To express indirectly (Merriam-Webster.com).

See also: Infinitive of → implication.

1a) To bring in (merchandise, commodities, workers, etc.) from a foreign country for use, sale, processing, reexport, or services.

1b) Computers: To bring (documents, data, etc.) into one software program from another.

2. The act of importing or bringing; something that is imported (Dictionary.com).

See also: From im-, variant of → in- + port, → export.

Of great significance or effect.

Etymology (EN): From M.Fr. important, from M.L. importantem, from importare “be significant in,” from importare “bring in,” → in- “into, in” + portare “to carry.”

Etymology (PE): Garin, on the model of Kurd. giring “important,” related to girân, gerân “heavy, important;” Mid.Pers. garân, Proto-Ir. stem *garu-; cf. Av. gouru-; Skt. guru- “heavy, important, venerable;” L. gravis “weighty, serious, heavy,” → gravity.

1. To lay on or set as something to be borne, endured, obeyed, fulfilled, paid, etc.
   

2. To put or set by or as if by authority (Dictionary.com).

Etymology (EN): M.E., from O.Fr. imposer “put, place; impute, charge, accuse,” from → in- “into, in” + poser “put, place,” → position.

Etymology (PE): Barnehâdan, literally “to place on, upon,” from bar-, → on-,

• nehâdan “to put, place,” → compose.

Very impressive because of great size, stately appearance, dignity, elegance, etc. (Dictionary.com).

See also: → impose; → -ing.

1. The laying on of something as a burden or obligation.
   

2. Something imposed, as a burden or duty; an unusual or extraordinarily burdensome requirement or task.
   

3. The act of imposing by or as if by authority (Dictionary.com).

See also: → impose; → -tion.

Not possible; unable to be, exist, happen. Unable to be done, performed, effected.

See also: → in-; → possible.

On who pretends to be what he is not.

Etymology (EN): M.Fr. imposteur, from L.L. impostor, agent noun from impostus, from imponere “to place upon, impose upon, deceive,” from → in- “into, in, on, upon” + ponere “to put place,” → position.

Etymology (PE): Daqalkâr, from daqal “imposture, deceit” + -kâr, agent noun suffix, from kardan, → -or.

Not precise; not exact; vague.

See also: From im-, variant of → in- + → precise.

The quality of lacking → precision.

See also: From im-, variant of → in- + → precision.

1. To press, to apply with pressure, especially to leave a mark.
   

2. To produce a profound effect upon the mind.

See also: → in-; → press.

1. A strong effect produced on the intellect, feelings, conscience, etc.
   

2. The first and immediate effect of an experience or perception upon the mind (dictionary.com).

See also: → press + -ion variant of → -tion.

Capable of being impressed; easily impressed.

See also: → impress; → -able.

1. A style of painting developed in the last third of the 19th century, characterized chiefly by short brush strokes of bright colors in immediate juxtaposition to represent the effect of light on objects.
   

2. A manner of painting in which the forms, colors, or tones of an object are lightly and rapidly indicated.
   

3. A manner of sculpture in which volumes are partially modeled and surfaces roughened to reflect light unevenly (Dictionary.com).

See also: → impression; → -ism.

Having the power to impress, to raise admiration, awe, respect, etc.

See also: → impress; → -ive.

Of a force acting on a body, the → product of the → force and the → time for which it acts. If the force changes with time, the impulse is the → integral of the force with respect to the time during which the force acts, and is
equal to the total change of → momentum produced by the force:

∫F dt = ∫m dv.

Impulse is a → vector quantity.

Etymology (EN): From L. impulsus “a push against, pressure, shock,” p.p. of impellere “to push, strike against, drive forward,” from → in- “into” + pellere “to push, drive.”

Etymology (PE): Tekâné, from tekân “involuntary motion, sudden shaking,” related to tak “rush, quick motion, stroke, blow” (tâxtan, tâzidan “to run; to hasten; to assault”); Mid.Pers. tak “assault, attack;” Av. taka- “leap, run,” from tak- “to run, flow;” cf. Skt. tak- “to rush, to hurry,” takti “runs;” O.Ir. tech- “to flow;” Lith. teketi “to walk, to flow;” O.C.S. tešti “to walk, to hurry;” Tokharian B cake “river;” PIE base *tek^w- “to run; to flow;” → flow.

The vector → impulse of the → resultant force on a particle, in any time interval, is equal in magnitude and duration to the vector change in momentum of the particle:

∫F dt = mv₂ - mv₁.

The impulse-momentum principle finds its chief application in connection with forces of short duration, such as those arising in collisions or explosions. Such forces are called → impulsive forces.

See also: → impulse; → momentum; → principle.

Relating to → impulse, the force which is very large but acts for a very short time.

See also: Impulsive, adj. of → impulse;
→ force.

A substance that is incorporated into a semiconductor material to provide free electrons (n-type impurity) or holes (p-type impurity).

Etymology (EN): Impurity, from im- negation prefix, → in-, + purity, O.Fr. pureté, from L.L. puritatem (nom. puritas) “cleanness, pureness,” from purus “clean;” cf. Av. pūitika- “serving for purification,” Mod.Pers. pâk “clean;” Skt. pavi- “to become clean,” pávate “purifies, cleanses;” O.H.G. fouwen, fewen “to sift;” PIE base *peu- “to purify, cleanse.”

Etymology (PE): Nâžâvi “impurity,” from nâ- negation prefix, → in-, + žâv “pure” + -i noun suffix.

The condition which exists when two waves of the same frequency pass through their maximum and minimum values in a correlated or synchronized way.

Etymology (EN): → in; → phase.

Etymology (PE): Hamfâz, from ham- “together,” → com- + fâz→ phase.

Situated in the natural or original position or place.

Etymology (EN): From L. in, → in-; L. situ, → site.

Etymology (PE): Dar “in,” → in-; jâ, → place.

1. Prefix meaning “into, in, on, upon, toward, at;” variants im-; il-; ir- by assimilation of -n- with the following consonant. It occurs also sometimes as en, in loans from O.Fr.
   
2. Prefix meaning “not, opposite of, without.”

Etymology (EN): 1) From L. in; cf. Gk. en; P.Gmc. *in (cf. O.Fris, Du., Ger., Goth. in); O.E. in, inne “within.”
2) From L. in- “not,” cognate with Gk. an-; O.Pers./Av. an-, a- “not, without;” Skt. an-, a- “not;” P.Gmc. *un-; O.E. un-.

Etymology (PE): 1) Dar- “in,” from Mid.Pers. andar, → intra-.

2. nâ-, na “not,” ma- “not” (prohibitive); Mid.Pers. nê, ma “no, not;” O.Pers. naiy, nai “not;” Av. nôit, naē “not;” cf. Skt. ná “not;” L. ne-, in-, un-; Gk. ni; Lith. nè; O.C.S. ne “not;” PIE *ne-.
   bi- privative prefix, from Mid.Pers. abi-, O.Pers. *apaiy-, Av. apa-.
   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.”

A group to which the speaker, the person spoken of, etc. belongs.

Etymology (EN): → in-; → group.

Etymology (PE): Xodi “insider, familiar, friend,” from xod “own,” → self-.

The quality or condition of being inaccurate.
The difference between the input quantity applied to a measuring instrument and the output quantity indicated by that instrument. The inaccuracy of an instrument is equal to the sum of its instrument error and its uncertainty.

See also: From negation prefix → in- + → accuracy

1. Not → active.
   

2. Chem.: Not → reactive.

See also: → in-; → active.

State of glowing at high → temperature; white or bright-red → heat.

See also: Quality, state noun from → incandescent.

1. (Of light) produced by → incandescence.
   

2. Glowing or → white with → heat.
   

3. Intensely → bright; brilliant (Dictionary.com).

Etymology (EN): From Fr., from L. incandescent-, incandescens, p.p. of incandescere “to become hot,” from → in- + candescere “to become hot,” from candere “to glow.”

Etymology (PE): Foruzandé, agent noun and verbal adj. from foruzidan, → glow.

Electric lamp in which light is produced by the heating effect of a → metal  → filament.

See also: → incandescent; → lamp.

1. The degree, extent, or frequency of occurrence, especially of something unwanted.
   

2. The act of striking a → light ray or → beam of particles on a surface. Also the direction of striking.

Etymology (EN): M.E., from M.Fr. incidence, from L.L. incidentia, from incidere “to happen, befall,” from in- “on” + -cidere, combining form of cadere “to fall,” → case.

Etymology (PE): Fotâd,literally “fall, the act of falling,” from fotâdan “to fall,” variant of oftâdan “to fall; to happen,” → coincide.

Same as → angle of incidence.

See also: → incidence; → angle.

1. An instance of something happening; an event or occurrence.
   

2a) Likely or apt to happen.

2b) Falling or striking on something, as light rays or a beam of particles.

Etymology (EN): → incidence.

Etymology (PE): Fotân, agent noun and adj. from fotâdan, → incidence.

The → light ray that strikes a surface before → reflection, → refraction, or → absorption. Opposite of → emergent ray.

See also: → incident; → ray.

To cut into; to cut marks upon.

Etymology (EN): M.E., from O.Fr. enciser, from L. incisus, p.p. of incidere “to cut into, cut through,” from → in- +
caedere “to cut,” → precise.

Etymology (PE): Darsunidan, from dar, → in- + sun “cut,” → precise.

The act of incising. A cutting into, especially for surgical purposes.

See also: Verbal noun of → incise.

To stir, encourage, or urge on; stimulate or prompt to action (Dictionary.com).

See also: → in-; → excite.

The act of inciting. The state of being incited. motive.

See also: Verbal noun of → incite.

1. The angle between two lines or two planes.
   
2. An orbital element that defines the angle between the orbital plane of a solar system body (planet, comet, asteroid) and the plane of the ecliptic.
   
3. The angle between the equatorial and orbital planes of a planet.

Etymology (EN): Inclination, from M.E. inclinacioun, from L. inclination-, from inclinat(us) p.p. of inclinare, from → in- + clinare “to bend;” cf. Gk. klinein “to cause to slope, slant, incline;” Skt. sri- “to lean,” O.Pers. θray-; Av. sray- “to lean;” Mod.Pers. kil “bent, inclined, slant;” PIE base *klei- “to lean, incline” from which is also derived
P.Gmc. *khlinen (Ger. lehnen, E. lean).

Etymology (PE): Darkil, from prefix dar- “in” + kil “bent, crooked, inclined” [Dehxodâ], Mid.Pers. xwahl “bent, crooked” (Mod.Pers. dialectal/colloquial variants kowleh, cowleh, cowl, caval, xohl, xohal, hol), cf. Skt. kûla- “slope, declivity;” PIE base *klei-, as above.

The angle between the rotation axis of a planet and the normal to the ecliptic.

See also: → inclination; → axis.

→ obliquity of the ecliptic.

See also: → inclination; → Earth; → equator.

To deviate from the vertical or horizontal; slant.

See also: Verbal form of → inclination.

To comprise as a part. See also → conclude, → exclude.

Etymology (EN): From L. includere “to shut in, enclose, imprison, insert,” from → in- “in” + claudere “to shut, close; to block up, make inaccessible,” ultimately from PIE root *klau- “hook, hooked branch or piece of wood; bolt, bar;” cf. L. clavis
“key,” clavus “nail,” claustrum “bar, bolt, barrier;” Gk. kleidos “bar, bolt, key;” O.Ir. clo “nail;” M.Ir. clithar “hedge, fence;” O.C.S. kljuci “hook, key,” kljuciti “shut;” Lith. kliuti “to catch, be caught on;” O.H.G. sliozan “shut,” Ger. schließen “to shut,” Schlüssel “key.”

Etymology (PE): Darkolândan, from dar-, → in-, + kolândan “to shut,” from kolân “wooden bolt, latch;” variants kelân, kolun, kolum, koland, kolandân, kelid, kelidân “wooden bolt, latch;” kalâk “a crook stick for gathering fruit;” (Tabari) kali, (Lori) kelil, (Laki) kalil “key; lock;” (Kurd.) kilil, kilij “key;” (prefixed) biškal(a) “a crooked stick for opening a lock, a sort of key;” (Kurd. Kurmanji) kelemce “handcuff;” (štiyâni) keleku “a large wooden mallet used by bleachers;” (prefixed) eškelak “a piece of wood used between a thief’s fingers to force him confess;” kor-mix, gol-mix “a large-headed peg or stake;” Mid.Pers. kilêl “key.” This family of Pers. words is probably related to PIE root *klau- “hook, hooked branch or piece of wood; bolt, bar,” as above.

1. The act of including; the state of being included; that which is included.
   

2. Fragment of foreign material enclosed within the mass of a → mineral or → meteorite. See also → CAI meteorite.

See also: Verbal noun of → include.

The lack of a fixed phase relationship between two or more waves. → coherent.

See also: Incoherent, from negation prefix → in- + → coherent; → wave.

1. The condition of two things being so different in nature as to be unable to exist together.
   

   2. Logic: (Of two or more propositions) unable to be true simultaneously.

See also: → incompatible; → -ity.

Of two or more propositions, unable to be true simultaneously. → compatible.

Etymology (EN): From M.L. incompatibilis, from → in- “not” + compatibilis (→ compatible).

Etymology (PE): Nâsâzgâr, from nâ- “not,” → in-,

• sâzgâr, → compatible.

The state of being incomplete, for example in photometric studies of a population of stars when the faintest members of the population are lacking. → completeness

See also: Negative of → completeness.

A flow whose volume or density does not change under pressure, and therefore its density is a constant. In other words, an ideal flow in which the → divergence of velocity is zero.
→ compressible flow.

See also: → incompressible, from → in “not” +
→ compressible; → flow.

1. Lack of consistency or agreement; incompatibility; an inconsistent feature or quality.
   

   2. Logic: The property of being inconsistent; a self-contradictory proposition (TheFreeDictionary.com).

See also: → in-; → consistency.

1. Lacking in consistency, agreement, or compatibility; at variance.
   

   2. Containing contradictory elements.
      

   3. Maths: (of two or more equations) not having one common set of values of the variables: x + 2y = 5 and x + 2y = 6 are inconsistent
      

   4. Logic: (of a set of propositions) Enabling an explicit contradiction to be validly derived (TheFreeDictionary.com).

See also: → in-; → consistent.

1. To make or become greater, in number, size, degree, etc..
   

2. The act of increasing. The amount by which something increases.

Etymology (EN): M.E., from O.Fr. encreiss-, pr.p. stem of encreistre, from L. increscere “to increase, to grow upon,” from → in- “in” + crescere “to grow,” → crescent.

Etymology (PE): Afzudan, from Mid.Pers. abzudan “to increase, grow,” O.Pers. abijav- “to increase, add to, promote,” from abi-, aiby- “in addition to; to; against” + root jav- “press forward,” Av. gav- “to hasten, drive,” Sk. jav- “to press forward, impel quickly, excite,” javate “hastens.”

The amount by which a varying quantity increases between two of its stages. In → calculus, the Greek letter Δ (delta) is used to denote an increment; e.g. Δx is an increment of x, contrasted to → decrement.

Etymology (EN): Increment, from L. incrementum “growth, increase,” from stem of increscere “to grow in or upon” from → in- “in” + crescere “to grow.”

Etymology (PE): Fazâyé, from fazâ shortened present stem of afzudan “to add, increase” (Mid.Pers. abzudan “to increase, grow;” O.Pers. abijav- “to increase, add to, promote,” from abi-, aiby- “in addition to; to; against” + root jav- “press forward;” Av. gav- “to hasten, drive;” Skt. jav- “to press forward, impel quickly, excite,” javate “hastens”) + -y- euphonic infix + -é noun/nuance suffix.

Not → definite; without fixed or specified limit.

See also: From → in- “not, without” + → definite.

Math.: An integral without upper and lower limits. The general antiderivative of
a function. → definite integral.

See also: → definite; → integral.

Not dependent.

See also: From negation prefix → in- + → dependent.

Statistics: Two events if the occurrence of one of them gives
no → information about whether or not the other event will occur; these events have no influence on each other.

See also: → independent; → event.

Statistics: Two random variables X and Y
if and only if the value of X has no influence on the value of Y and vice versa.

See also: → dependent; → random; → variable.

Math.: A variable whose value determines the value of other variables.
For example, x, in the expression y = f(x), is the independent variable.
See also → dependent variable.

See also: → independent; → variable.

Philosophy: The doctrine that there are events which do not correspond with determinism and therefore are uncaused in some sense.
Quantum physics: The claim that the most basic constituents of matter behave indeterministically, in accordance with such properties as the → uncertainty principle.

Etymology (EN): → in-; → determinism.

Etymology (PE): Nâtarmbâvari, nâtarmvari, from nâ- negation prefix

• (â)tarmbâvari, (â)tarmvari, → determinism.

1. Math.: A number or symbol, often written as a → subscript or → superscript to a mathematical expression, that indicates an operation to be performed, an ordering relation, or a use of the associated expression.
   

2. Physics: A measure of a particular property; e.g. → adiabatic index; → color index; → index of refraction.
   

3. In some → astrolabes, the → rule.
   

4. A sequential arrangement of items, especially in alphabetical or numerical order. → Index Catalogue.

Etymology (EN): Index, from L. index “forefinger, pointer, sign,” literally
“anything which points out,” from indicare “point out, show,” from in- “in” + dicare “to proclaim,” from stem of dicere
“to speak, to say;” PIE base *deik- “to point out” (cf. Av. daēs- " to show;" Skt. dic- “to point out, show;” Gk. deiknynai “to prove;” O.H.G. zeigon; Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach”).

Etymology (PE): 1) Dišan, from diš-, simple aorist of Av. daēs- “to show,” as above, + suffix -an.
2) Fehrest, from Mid.Pers. payrâstag “arranged,” from payrâstan “to arrange; to adorn” (Mod.Pers. pirâstan); Proto-Ir. *pati-rāsta-, from *pati “to, at, for, in” (cf. Mid.Pers.
pad “to, at, for, in;” Mod.Pers. be, beh;
Old.Pers. paity; Av. paiti “to, toward, in, at;”
Skt. práti; Gk. poti) + *rād- “to direct, arrange”
(cf. Mod.Pers. raj “line, row,” variants raž, rak, râk, rezg (Lori), ris, risé, radé, rasté, râsté, related to râst “right, true; just, upright, straight;” Mid.Pers. râst “true, straight, direct;” Sogdian rəšt “right;” O.Pers. rāsta- “straight, true,” rās- “to be right, straight, true;” Av. rāz- “to direct, put in line, set,” razan- “order;”
cf. Skt. raj- “to direct, stretch,” rjuyant- “walking straight;” Gk. orektos “stretched out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” PIE base *reg- “move in a straight line,” hence, “to direct, rule”).

Either of two catalogues of non-stellar objects, which serve as supplements to the → New General Catalogue.

See also: → index; → catalog

Same as → refractive index.

See also: → index; → refraction.

The → numeral system consisting of the numerals 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 which evolved into the system we use today. The Indian numerals was a place-value or positional system. The Indians were the first to develop a base 10 positional system. Same as → Hindu-Arabic numeral system.

See also: Indian, from India, → Indus; → numeral; → system.

A color between blue and violet.

Etymology (EN): From Sp. indico, Portuguese endego, and Du. (via Portuguese) indigo, from L. indicum “indigo,” from Gk. indikon “blue dye from India,” literally “Indian (substance),” neuter of indikos “Indian,” → Indus.

Etymology (PE): Nil “indigo color; the indigo plant;” Mid.Pers. nil; cf. Skt. nīlī, nīlā “the indigo plant; indigo color.”

Not in a direct course, path, action, or procedure.

See also: → in-; → direct.

A word or group of words representing the person or thing with reference to which the action of a verb is performed. In English generally coming between the verb and the direct object and paraphrasable as the object of a preposition, usually to or for, following the direct object, as the boy in He gave the boy a book (Dictionary.com).

See also: → indirect; → object.

1a) A single human being, as distinguished from a group (Dictionary.com).

1b) A distinct, indivisible entity; a single thing, being, instance, or item
(Dictionary.com).

1c) Logic: An → object or element taken as the → subject of the → predicates of → first-order logic.

2. Of, pertaining to, or characteristic of a particular person or thing. → individual constant, → individual variable.

Etymology (EN): M.E., from M.L. individualis, from L. individuus “indivisible,” from → in- “not, opposite of” + dividuus “divisible,” from dividere, → divide.

Etymology (PE): Takâl, from tak, → single, + suffix -âl, → -al; takâli with adj. suffix -i.

In predicate logic: A → word or → expression that represents a → specific  → individual or → object. A single object can be denoted by multiple individual constants, reflecting the fact that objects can have multiple names. On the other hand, any individual constant can only denote one object. Individual constants are represented by lower case letters from the beginning of the alphabet: a, b, c.

See also: → individual; → constant.

In → predicate logic,
a variable which represents an unspecified → subject. Individual variables work just as do variables in algebra, they stand in for a value, but have no fixed value of their own. They are represented by lower case letters taken from the end of the alphabet x, y, z; and if additional variables are needed by u, v, w, as well.

See also: → individual; → variable.

1. A social theory advocating the liberty, rights, or independent action of the individual (Dictionary.com).
   

2. An individual peculiarity.

See also: → individual; → -ism.

1. A person who shows great independence or individuality in thought or action (Dictionary.com).
   

2. An advocate of → individualism (Dictionary.com).

See also: → individual; → -ist.

The particular character, or aggregate of qualities, that distinguishes one person or thing from others (Dictionary.com).

See also: → individual; → -ity.

The process or state of discriminating the individual from the generic group or species.

See also: → individualize; → -tion.

To make individual or distinctive; give an individual or distinctive character to (Dictionary.com).

See also: → individual; → -ize.

To cause or initiate by influence or some means; e.g. → induced current; → induced emission.

Etymology (EN): Induce, “to lead by persuasions or other influences,” from L. inducere “to lead into, persuade,” from → in- “in”

• ducere “to lead,” from PIE *deuk- “to lead” (cf. O.E. togian “to pull, drag,” O.H.G. ziohan “to pull”).

Etymology (PE): Darhâxtan, darhâzidan, from dar- “in” + Mid.Pers. hâxtan, hâzidan, “to lead, guide, persuade;”
Av. hak-, hacaiti “to attach oneself to, to join;” cf. Skt. sacate “accompanies, follows;” Gk. hepesthai “to follow,"; L. sequi “to follow;” PIE *sek^w- “to follow.”

Brought about or caused by → induction.

See also: Induced, p.p. of → induce.

Current resulting from the relative motion of a conductor through a magnetic field.

See also: → induced ; → current.

An electric field created by the variation of a magnetic field. The induced electric field lines are usually perpendicular to the changing magnetic field that produces them.

See also: → induced; → electric; → field.

The emission of radiation from an atom when it is bombarded by photons. The induced radiation has the same wavelength and direction as the bombarding radiation. Same as → stimulated emission.

See also: → induced; → emission.

Fission which takes place when a nucleon is bombarded with neutrons or high-energy radiation (gamma rays).

See also: Induced, p.p. of → induce; → fission.

The property of an electric circuit, or of two neighboring circuits, by virtue of which an electromotive force is induced in one circuit by a change of current in either of them.

See also: Inductance, from induct, from L. inductus, p.p. of
→ induce + → -ance.

1. General: The act of inducing, bringing about, or causing.
   

2. Electromagnetism: A common term for the process by which a body having electric or magnetic properties produces magnetism, an electric charge, or an → electromotive force in a neighboring body without contact. For more details, see → electromagnetic induction; → electrostatic induction; → magnetic induction.
   

3. Math.: A method of mathematical proof typically used to establish that a given statement is true of all natural numbers. It is done by proving that the first statement in the infinite sequence of statements is true, and then proving that if any one statement in the infinite sequence of statements is true, then so is the next one.
   

4. Logic: Any form of reasoning in which the conclusion, though supported by the premises, does not follow from them necessarily. → inductive reasoning.

See also: Verbal noun of → induce.

A device for producing high-voltage pulses by means of → electromagnetic induction. It consists of a primary coil of a few turns of wire, wound on an iron core, and insulated from a secondary coil of many turns which surrounds it coaxially. The current in the primary, which is interrupted periodically, sets up a magnetic field, first big, then zero. This changing field induces a large voltage in the secondary.

See also: → induction; → coil.

In magnetohydrodynamics, an equation that describes the transport of plasma and magnetic field lines over time:
∂B/∂t = ∇ x (v x B) + η∇²B,

where B is the → magnetic induction, v is the plasma velocity, and η = (μσ)^-1 the → magnetic diffusivity. The first term on the right side represents → magnetic advection and the second term → magnetic diffusion.
The induction equation can also be expressed as:
∂B/∂t = -(v.∇)B + (B.∇)v - B(∇.v),

where the terms of the right-hand side stand for advection, stretching, and compression, respectively. Among these terms, net increase of the field can be done only through the stretching and compression.

See also: → induction; → equation.

A component of an electromagnetic field which is
the region close to the source (an antenna) where steady-state magnetic and electrostatic forces can be detected. → radiation field.

See also: → induction; → field.

Of, relating to, or using → induction. induction

See also: From induct, → induce, + → -ive.

Reasoning from detailed facts to general principles.

See also: → deductive; → reasoning.

A coil of wire used to introduce inductance into an electric circuit.

See also: Agent noun of → induce.

The American Indian. A faint, southern constellation (right ascension: 21 hours, declination: -55 degrees), supposed to represent an American native Indian. It was introduced in the 1603 star atlas of Johann Bayer and contains no stars brighter than the third magnitude. Abbreviation: Ind; genitive: Indi.

Etymology (EN): Indus “an Indian,” from L., from Gk. Indos “the Indus River, an Indian.”

Etymology (PE): Sorxpust “red skin,” the term coming from the reddish skin color of some native Americans, from sorx, → red, + pust “skin,” from Mid.Pers. pôst “skin;” Av. pastô-, in pastô.fraθanhəm “of the breadth of the skin.”
Hendi “Indian,” Mid.Pers. Hindûk, from Hind “Indian;” O.Pers. hindu- “Sind, a province of the Persian Empire on the upper Indus River,” Hinduya- “man of Sind;” Av. hindu-, həndu-; Skt. sindhu- “stream; the Indus; country around the Indus.”

A collision between bodies in which there is a loss of total kinetic energy.

See also: → inelastic; → collision.

A type of scattering when the → scattered radiation has a → wavelength different from that of the → incident radiation (→ Raman scattering, → fluorescence ).

See also: → inelastic; → scattering.

1. A statement of the form a ≠ b, a > b, or a < b, asserting one quantity is greater than or less than another quantity. → equality.
   
2. An irregularity in the movement of a celestial object in its orbit about another which cannot be explained by their mutual gravitational attraction. Irregularities are often due to perturbation by other neighboring objects.

See also: → in-; → equality.

Having no inherent power of action, motion, or resistance (opposed to active).
Chemistry: A chemically inactive element, compound or material. → inert gas.

Etymology (EN): Inert, from Fr. inerte, from L. inertem “unskilled, inactive,” from → in- “without” + ars (gen. artis) “skill.”

Etymology (PE): Laxt “sluggish, inert.”

Any one of six gases helium, neon, argon, krypton, xenon, and radon, all of whose atomic shells contain complete numbers of electrons so that the atoms are almost completely chemically inactive. Same as noble gases.

See also: → inert; → gas.

Tendency of a body to preserve its state of rest or uniform motion in a straight line.

Etymology (EN): Inertia, from L. inertia “un-skillfulness, idleness,” from iners (gen. inertis) “unskilled, inactive;” → inert.

Etymology (PE): Laxti “sluggishness, inertia.”

An ellipsoid used in describing the motion of a rotating rigid body. It is stationary with respect to the rotating body, and is determined by the body’s moments of inertia.

See also: → ellipsoid; → inertia.

Of or relating to inertia.

Etymology (EN): From → inertia + → -al.

Etymology (PE): Laxt, adj. of laxti, → inertia

A force arising from the → acceleration of an observer’s → frame of reference.

See also: → inertial; → force.

→ inertial reference frame.

See also: → inertial; → frame.

The mass of a body as determined from the acceleration of the body when it is subjected to a force that is not due to gravity.

See also: → inertial; → mass.

Motion free of any force, with constant velocity.

See also: → inertial; → motion.

1. A periodic motion of a particle that moves, free from external forces, over the surface of a rotating sphere, such the Earth. Inertial oscillations result from the → Coriolis force. For example, a hockey puck launched on a big enough lake in the northern hemisphere would turn to the right (east) and eventually loop back to nearly the initial point (actually west of that point). The time it takes for the huckey puck to return can be computed with the → Coriolis frequency.
   

2. Meteo.: An anticyclonic (clockwise) rotation in the northern hemisphere, with the Coriolis force providing the → centripetal acceleration with period of 2π/f, where f is the Coriolis frequency. The opposite should occur in the southern hemisphere.

See also: → inertial; → oscillation.

A → reference frame or coordinate system in which there are no accelerations, only zero or uniform motion in a straight line. According to the special theory of relativity, it is impossible to distinguish between such frames by means of any internal measurement.

See also: → inertial; → reference; → frame.

The fall of matter toward the center of a region of gravitational influence.

Etymology (EN): Infall, from → in- + → fall.

Etymology (PE): Darun-oft, from darun- “in, into; within” (Mid.Pers. andarôn
“inside,” from andar, → inter-, + rôn “side, direction;” Av. ravan- “(course of a) river”)

• oft, → fall.

To affect, cause infection, or contaminate with a pathogenic microorganism or agent.

Etymology (EN): M.E. infecten, from L. infectus, p.p. of inficere “to immerse in dye, discolor, taint, poison,” from equivalent to → in- + → -fac, combining form of facere “to do, make,” → fact, + -tus p.p. suffix; from PIE root *dhe- “to set, put.”

Etymology (PE): Fažidan, infinitive from faž “dirt, filth, pus;” related word faže, fažnâk, fažgen, fažegen, fažgin, fažqand “filthy, dirty;” of unknown origin. It may be related to Proto-Ir. root *xard- “to defecate,” with offshoots xard “muddy place,” xarra “mud,” (+ *pati-) payxâl “excrement.”

1. A thing that infects or causes infection.
   

   2. Causing infection; infecting.

See also: → infect; → -ant.

1. Affected with a disease-causing organism.
   

   2. Computers: Affected with a → virus.

See also: P. p. of → infect.

A branch of medicine that deals with the diagnosis, management and treatment of various infectious or contagious diseases.

See also: → infection; → -logy.

1. An act or fact of infecting; state of being infected.
   

   2. An infecting agency or influence.
      

   3. An infectious disease.
      

   4. Computers: The presence of a virus in, or its introduction into, a computer system.

See also: Verbal noun of → infect.

1. Capable of causing infection.
   

2. Caused by or capable of being transmitted by infection.

See also: → infect(ion) + -ious, variant of → -ous.

Capable of causing infection.

See also: → infect; → -ive.

1. The act or process of deriving logical conclusions from premises known or assumed to be true.
   
2. Something that is inferred.

Etymology (EN): From M.L. inferentia, from inferre, from → in- “in” + ferre “to carry, bear,” cognate with Pers. bordan “to carry, bear,” as below.

Etymology (PE): Darbord (on the models of daryâft “perception” and peybord “understanding, finding;” see also bâzbord, → reference), from dar-, → in-,

• bord past stem of bordan “to carry, bear;” (Mid.Pers. burdan; O.Pers./Av. bar- “to bear, carry,” Av. barəθre “to bear (infinitive);” Skt. bharati “he carries;” Gk. pherein; L. fero “to carry;” PIE base *bher- “to carry”).

Lower in place or position; closer to the bottom or base.
Astro.: → inferior conjunction;
→ inferior culmination; → inferior planet.

Etymology (EN): From L. inferior “lower,” comp. form of inferus (adj.) “that is below or beneath,” from infra “below” (→ infrared), cognate with Pers. zir, as below.

Etymology (PE): Zirin, adj. from zir “below, down,” Mid.Pers. azêr “below, under,” êr “below, down; low, under,” adar “low;”
Av. aδara- (adj.), aδairi- (prep.) “below;” cf. Skt. ádhara- “lower;” L. infra (adv., prep.) “below, underneath, beneath,” inferus “lower;” O.E. under “under, among”);
PIE base *ndher.

The conjunction of an inferior planet with the Sun when the planet is between the Sun and the Earth. → superior conjunction.

See also: → inferior; → conjunction.

The meridian transit of a star between the celestial pole and the north point of the horizon. Same as → lower culmination. → superior culmination.

See also: → inferior; → culmination.

A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in the Solar System.

See also: → inferior; → planète.

Unlimited or unmeasurable in extent of space, duration of time, etc.
Math.: Not finite; an infinite quantity or magnitude; large beyond bound.
Of a set, having elements that can be put into one-to-one correspondence with a subset that is not the given set.

See also: → in- + → finite.

A → statistical population consisting of individuals or items which either possesses the infinite property through some limiting process or is non-enumerable. For example, the population of all → real numbers between 0 and 1 and the population of all → integers are examples of infinite population. In case of random sampling with replacement, any population is always infinite.

See also: → infinite; → population.

A series with infinitely many terms, in other words a series that has no last term, such as 1 + 1/4 + 1/9 + 1/16 + · · · + 1/n² + … .

The idea of infinite series is familiar from decimal expansions, for instance the expansion π = 3.14159265358979… can be written as π = 3 + 1/10 + 4/10² + 1/10³ + 5/10⁴ + 9/10⁵ + 2/10⁶ + 6/10⁷ + 5/10⁸ + 3/10⁹ + 5/10¹⁰ + 8/10¹¹ + … , so π is an “infinite sum” of fractions. See also → finite series.

See also: → infinite; → series.

A set which can be put in a one-to-one correspondence with part of itself.

Etymology (PE): → infinite; → set.

General: Indefinitely or exceedingly small.
Math.: A variable that approaches zero as a limit. A quantity decreasing indefinitely without actually becoming zero.

Etymology (EN): Infinitesimal, coined by Ger. philosopher and mathematician Baron Gottfried Wilhelm von Leibniz (1646-1716) from N.L. infinitesim(us) “infinite in rank,” from infinit(us), → infinite, + -esimus suffix of ordinal numerals + → -al.

Etymology (PE): Bikarânxord, from bikarân “unbounded, unlimited, infinite,”
from bi- “without” + karân “boundary, side, end” (variants karâné, kenâr, from Mid.Pers. karân, karânak, kenâr “edge, limit, boundary,” Av. karana- “side, boundary, end”)

• xord “minute, little, small” (from Mid.Pers. xvart, xôrt “small, insignificant;” Av. ādra- “weak, dependent;” Skt. ādhrá- “small, weak, poor,” nādh “to be oppressed;” Gk. nothros “sluggish;” PIE base *nhdhro-).

The body of rules and processes by means of which continuously varying magnitudes are dealt with in → calculus. The combined methods of mathematical analysis of → differential calculus and → integral calculus.

See also: → infinitesimal, → calculus.

The nominal form of the verb which expresses the idea of action or state without indicating person, number, or tense.

Etymology (EN): Contraction of L. infinitivus modus “unlimited, indefinite mood,” from L. infinitus, → infinite.

Etymology (PE): Karpâyé, literally “action base,” from kar-, → verb, + pâyé, → base.

That quantity which is greater than any assignable quantity.

See also: Noun form of → infinite.

1. Feeble or weak in body or health, especially because of age.
   

2. To unvalidate.

Etymology (EN): M.E. infirm, from L. infirmus “weak, frail, not strong or firm” from → in- “not, opposite of” + firmus, → firm.

Etymology (PE): Nâdeš, from nâ- “not,” → in-, + deš, → firm.

An → affix that is inserted within a root or stem.

Etymology (EN): From L. infixus p.p. of infigere “to fasten in,” from in-

• figere “fasten.”

Etymology (PE): Darvand, from dar- “in” + vand, → affix.

To become inflated; to increase, especially suddenly and substantially. → inflation, → inflatory model.

Etymology (EN): Inflate, from L. inflatus p.p. of inflare “to blow into, puff up,” from → in- “into” + flare “to blow.”

Etymology (PE): Pandâmidan “to swell,” from pandâm [Mo’in] “swelling;” Borujerdi panâm, panam “swellig;” Malâyeri panomidan “to swell;” Laki penamiyen “to swell;” Hamadâni pandumidan “swelling of the eye or other parts of the body;” Kermâni padum kerdan “to swell,” padum “swelled; fat, corpulent;” Tâleši pandâm, pandom “swelling;” Gilaki pandâm kudan “rising of river water caused by flood;” cf. Gk. pneuma “wind; breath,” from pnein “to blow; to breathe;” PIE base *pneu- “to breathe.” Related terms in other Indo-European languages: O.E. fnaeran “to breathe heavily,” fneosan “to snort, sneeze;”
M.H.G. pfnusen, pfnehen “to breathe, pant, sniff, snort, sneeze;” Norw. fnysa “to breeze;” M.Du. fniesen, Du. fniezen “to sneeze;”
O.H.G. niosan, Ger. niesen “to sneeze.”

1. General: The act of inflating; the state of being inflated.
   

2. Cosmology: A brief exponential expansion of the Universe postulated to have occurred 10^-35 seconds after the → Big Bang, in response to the separation of the → strong interaction from the → electroweak interaction. This idea aims at explaining the → flatness problem, the → horizon problem, and the → magnetic monopole problem. See also → inflaton field.

See also: Verbal noun of → inflate.

A class of → Big Bang models of the Universe that include a finite period of accelerated expansion in their early histories. Such an event would have released enormous energy, stored until then in the vacuum of space-time. The horizon of the Universe expanded, temporarily, much faster than the speed of light.
→ inflaton field.

See also: Inflationary, adj. of → inflation; → model.

The hypothetical → particle that mediates the hypothetical → inflaton field.

See also: From inflat-, from → inflaton field, + particle suffix → -on.

A hypothetical → scalar field that provides a theoretical basis for → inflation in the early → Big Bang history of the → Universe. The inflaton field would fill space with the same energy at every point. In general, the scalar field can vary with time and space, though to a first approximation everywhere in the Universe will have the same value at any time. The field has a particle associated with it, called → inflaton, just as the → electromagnetic field is associated with the → photon. The inflaton field is characterized also by a → negative pressure that would yield a tremendous → repulsive gravity during a brief lapse of time. In the earliest moments of the Universe, space is uniformly filled with an inflaton field, whose value places it higher up on its → potential energy curve. The inflaton’s → potential energy would drop
in a tiny fraction of a second, on the order of 10^-35 seconds. And yet, during that brief instant, space would expand by a colossal factor, of at least 10³⁰.

See also: → inflaton; → field.

1. General: To bend in, to turn from a direct line or course.
   
2. Grammar: To change the ending or form of a word in accordance with other words in a sentence. To apply → inflection to.

Etymology (EN): From M.E. inflecten, from L. inflectere “to bend in, bow, curve,” figuratively, “to change,” from → in- + flectere “to bend, to curve,” of uncertain origin.

Etymology (PE): Darcaftidan, from dar-, → in-, + caftidan, → flex.

A change in the form of a word to indicate a change in such grammatical features as tense, person, gender, case, number, voice, or mood. A general term for → declension and → conjugation.

See also: Verbal noun of → inflect.

A point on a → curve at which the → tangent changes direction, from rotating in one sense to rotating in the opposite sense.

See also: → inflection; → point.

Of, relating to, or characterized by the use of → inflection, e.g.
→ inflectional affix.

See also: → inflection + → -al.

An → inflection that is added at the end of a root word. In English there are eight inflectional affixes, which are all suffixes. They always follow derivational suffixes and do not change the category of a word.

See also: → inflectional + → affix.

1. The act or process of flowing in or into. Something that flows in or into. Opposite of → outflow.
   

2. Meteo: Flow of water into a stream, lake, reservoir, container, basin, aquifer system, etc.

See also: Inflow, from → in- + → flow.

1. The act or power of producing an → effect, especially inconspicuously; the effect of power exerted.
   

2. To have or exert influence on; affect.

Etymology (EN): M.E., from O.Fr. influence, from L. influentia “stellar emanation” (it was thought an ethereal liquid flowed from the stars and affected the destiny of humans), also “a flow of water,
a flowing in,” from L. influentem (nominative influens), pr.p. of influere “to flow into,” from → in- “into, in, on, upon” + fluere “to flow;” PIE base *bhleu- “to swell, overflow;” cf. Gk. phluein “to boil over, bubble up,” phlein “to abound.”

Etymology (PE): Hanâyeš “influence” (Borhân-e Qâte’; Kasravi). We suggest the following origin
for this word: ultimately from Proto-Ir. *ha-nai “to conduct, direct, guide, lead,”
from *ham-naiH-, from prefix *ham- “together,” also an intensive prefix, → com-, + *naiH- “to lead, conduct, guide;” cf. Av. naii- (naŷ-) “to lead, guide,” naiieiti “leads;”
O.Pers. nay- “to lead, bring” anaya “leads;” Mid.Pers. nidan, ônidan, nay- “to lead, bring;” Sogd. n’y- “to lead;” Choresmian (prefixed, + *apa-) bny- “to remove;” Skt. nī- “to lead,” náyati “leads.”
In dialects: Anâraki niye “to bring, lead;” Kurd. (prefixed) ânin, inân, (with prefix ham-) hânin, hênân “to bring, lead to,” Tâleši ânân, ânoe “to bring together two edges;” Zazaki ân- “to bring;” Qohrudi hunda, hun- “to bring, lead;”
Târi niya-, neg- “to lead;” Lasgardi (prefixed) bin- “to bring, carry;” Qomi niyé, Tafreši neyé “a jab or poke used for leading the cattle;” see also → relate.

1. To impart → knowledge of a fact or circumstance.
   

2. To supply (oneself) with knowledge of a matter or subject (dictionary.com).
   

3. To convey → information.
   

See also: → informatics, → information theory.

Etymology (EN): Inform, from M.E. informen, from O.Fr. enformer, from L. informare “to shape, form; to form an idea of,” from → in- “into” + → forma “form.”

Etymology (PE): Azdâyidan, from Mid.Pers. azdênidan “to inform,” from azd “information, proclamation; known” (loaned into Arm. azd), from O.Pers. azdā “known,” azdā kar “to become known;” Av. azdā “known;” Sogdian ‘zd’ “informed, known;” cf. Skt. addhā’ “manifestly; certainly, truly,” addhāti “sage.”

A person who gives → information.

See also: From → inform + -ant a suffix forming adjectives and nouns from verbs.

The science concerned with gathering, manipulating, storing, retrieving, and classifying recorded → information. Also called → information science, computer science.

See also: Informatics, from informat(ion), → information + → ics.

1. Knowledge gained through study, communication, research, instruction, etc.
   

2. Knowledge communicated or received concerning a particular fact or circumstance; news.
   

3. The act or fact of informing.
   

4. A sequence of signals that conveys a message to a receiver. Information does not exist on its own. It is contained within something and is coded. The notion of information involves → uncertainty and → randomness; hence the necessity for calling on → probability theory and → statistics.
   

See also:

→ information content, → information entropy, → information flow, → information paradox, → information science, → information technology, → information theory, → infosphere, → quantum information, → entropy.

See also: Verbal noun of → inform.

The → negative of the → logarithm of the → probability that a particular
→ message or → symbol will be emitted by a → source.

See also: → information; → content.

The measure of information, which is usually expressed by the average number of bits needed for storage or communication. In other words, the degree to which the values of a → random variable X
are dispersed. If the → probability density function of X is P(x), the entropy is defined by: H(X) = -Σ P(x) log P(x). Also called → Shannon entropy.

See also: → information; → entropy.

The flow of data into a system or to the end users.

See also: → information; → flow.

A paradox raised in 1976 by S. Hawking (1942-2018) whose analysis of the thermodynamic properties of → black holes led him to the prediction that black holes are not in fact black, but radiate due to quantum effects. This implied that, due to the → Hawking radiation, a black hole would eventually evaporate away, leaving nothing. This deduction presented a problem for → quantum mechanics, which maintains that information can never be lost.

This topic is a matter of intense debate. Many solutions have been proposed, but all of them have serious drawbacks. In order to analyze better these solutions one needs a quantum gravity theory, which does not exist at the moment.

In brief, either the idea of → quantum unitarity must be given up, or a mechanism should be found by which information is not lost after it falls into a black hole.

See also: → information; → paradox.

Same as → informatics.

See also: → information; → science.

The science and activity of receiving, storing, processing, and transmitting information by using → computers.

See also: → information; → technology.

The mathematical theory that defines, quantifies, and analyzes the concept of → information. It involves → probability theory in → transmission of → messages when the → bits of information are subject to various distortions. Its goal is to enable as much information as possible to be reliably stored on a medium, retrieved, or communicated.

See also:
→ information content, → information entropy, → information flow, → information paradox, → information science, → information theory, → quantum information, → entropy.

See also: → information; → theory.

Giving → information, providing information, imparting → knowledge.

See also: From informat-, from L. informatus, from → inform + → -ive.

A person who provides → information.

See also: From → inform + -er, → -or.

The global network of all the world’s communications, databases, and sources of information.

See also: → information; → sphere.

A prefix meaning “below,” used with second elements of any origin.
→ infrared; → infrasound.

Etymology (EN): From L. infra “below, underneath, beneath,” from PIE *ndher; cf. Mod.Pers. zir “below, down,” akin to → under-.

Etymology (PE): Foru-, from foru “down, downward; below; under, beneath; low,” from Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain.”

The invisible part of electromagnetic spectrum possessing wavelengths between those of visible light and those of radio waves, i.e. approximately between about 0.75 and 1000 μm.

Etymology (EN): From L. infra (adv., prep.) “below, underneath, beneath,” inferus “lower,” from PIE *ndher; cf. Av. aδara- (adj.), aδairi- (prep.) “below;” Mid.Pers. êr “below, down; low, under,” adar “low,” azêr “below, under” (Mod.Pers. zir “below, down”); Skt. ádhara- “lower;” O.E. under “under, among”); → red.

Etymology (PE): Forusorx, from foru “down, downward; below; beneath;” Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain,” + sorx→ red.

A two-dimensional infrared imaging device, consisting of an array of small, individual electronic detectors, each of which records a pixel in the image.

See also: → infrared; → array.

An orbiting infrared telescope (60 cm mirror) which successfully operated from launch
in January 1983 until the supply of coolant ran out in November 1983. It was a collaborative mission between NASA, the Netherlands, and the UK, and mapped 95% of the whole sky in the wavelength bands 12, 25, 60, and 100 microns.

See also: → infrared; → astronomical; → satellite.

The study of infrared properties of astronomical objects.

See also: → infrared; → astronomy.

An optical camera equipped with a → detector which is sensitive to → infrared radiation.

See also: → infrared; → camera.

Wispy, extended structures seen above and below the plane of the Galaxy in the infrared maps of the sky (60-100 microns) produced by the → Infrared Astronomical Satellite (IRAS).

See also: → infrared; L. cirrus “curl, tuft, plant filament like a tuft of hair.”

A → dark cloud characterized by a → visual extinction A_v≥ 10² mag. IRDCs are opaque even at 8 μm, and can be seen in silhouette against the bright diffuse → mid-infrared emission in the → interstellar medium.

See also: → infrared; → dark; → cloud.

A thermal device for observing and measuring → infrared radiation.

See also: → infrared; → detector.

The portion of → electromagnetic radiation from → astrophysical objects in → infrared frequencies.

See also: → infrared; → emission.

Infrared → emission from a star in excess of that expected from a → blackbody with the temperature corresponding to the temperature of the star.

See also: → infrared; → excess.

A galaxy that emits most of its energy in the infrared region of the spectrum. Such galaxies are thought to have unusually high rates of star formation and are also described as → starburst galaxies.

See also: → infrared; → galaxy.

Imaging with an infrared detector.

See also: → infrared; → imaging.

That part of the → electromagnetic radiation lying beyond the red, between the radio and the visible regions of the → electromagnetic spectrum. The wavelengths range from about 0.8 → microns (μm) to about 1000 μm.

See also:

→ near-infrared; → mid-infrared; → far-infrared; → submillimeter radiation.

See also: → infrared; → radiation.

A European Space Agency satellite which carried the most sensitive infrared telescope ever launched. It operated between November 1995 and April 1998 and made particularly important observations of the dusty regions of the Universe.
ISO was equipped with four science instruments: an infrared camera (CAM), a long-wavelength spectrometer (LWS), a photo-polarimeter (PHT), and a short-wavelength spectrometer (SWS). The instruments jointly covered wavelengths from 2.5 to around 240 microns with spatial resolutions ranging from 1.5 arcseconds to 90 arcseconds. Its 60 cm diameter telescope was cooled by
superfluid liquid helium to temperatures of 2-4 K. The mission was a great technical, operational and scientific success. During its routine operational phase, ISO successfully made some 30,000 individual imaging, photometric, spectroscopic, and polarimetric observations ranging from objects in our own solar system to the most distant extragalactic sources.

See also: → infrared; → space; → observatory.

Observing a large area of sky (or the whole sky) in infrared wavelengths.

See also: → infrared; → survey.

A telescope capable of observing → infrared radiation from astronomical objects.

See also: → infrared; → telescope.

A range of infrared wavelengths to which the Earth’s atmosphere is relatively transparent, and at which observations can be made from the ground. Infrared windows are found near wavelengths of 1.25, 1.65, 2.2, 3.6, 5.0, 10, 20, and 30 microns, and beyond 300 microns.

See also: → infrared; → window.

A sound wave whose frequency is below the audible range of about 20 → Hz to 20,000 Hz. → sound wave; → ultrasound; → subsonic.

See also: → infra- + → sound.

That which enters into a → compound or → mixture.

Etymology (EN): M.E., from L. ingredient- (nominative ingrediens) “that which enters into,” p.p. of ingredi “to go in, enter,” from → in- + gradi “to go, to walk,” → egress.

Etymology (PE): Darâné, literally “brought into,” from dar-, → in-, + ân present stem of ânidan “to bring, to lead,” → relate, + nuance suffix -é.

Same as → immersion.

See also: From → in- + gress, → egress.

To take or receive (property, a right, a title, etc.) by succession or will, as an → heir (Dictionary.com).

See also: → heritage.

1. Capable of being inherited.
   

2. Capable of inheriting; qualified to inherit (Dictionary.com).

See also: → inherit; → -able.

1. Something that is or may be inherited; property passing at the owner’s death to the heir or those entitled to succeed; legacy; something, as a quality, characteristic, or other immaterial possession, received from progenitors or predecessors as if by succession.
   

2. The genetic characters transmitted from parent to offspring, taken collectively.
   

3. The act or fact of inheriting by succession, as if by succession, or genetically (Dictionary.com).

See also: Verbal noun of → inherit.

A person who → inherits; → heir.

See also: → inherit; → -or.

The condition or an instance of not being homogeneous.

See also: → in- “not” + → homogeneity.

Lack of homogeneity; something that is not → homogeneous.

See also: → in- “not” + → homogeneous.

Of, pertaining to, or occurring at the beginning.

Etymology (EN): Initial, from L. initialis, from initium “a beginning, an entrance,” from p.p. stem of inire “to go into, begin,” from → in- + ire “to go,” → ion.

Etymology (PE): Âqâzin “pertaing to the beginning,” from âqâz “beginning,” from Proto-Iranian *āgāza-, from prefix ā-

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

1. Conditions at an initial time t = t₀ from which a physical system or a given set of mathematical equations evolves.
   

2. Meteo.: A prescription of the state of a → dynamical system at a specified time; for all subsequent times, the → equation of motion and → boundary conditions determine the state of the system.

See also: → initial; → condition.

The mass of a star at its arrival on the → main sequence.

See also: → initial; → mass.

A mathematical expression describing the relative number of stars found in different ranges of mass for a cluster of stars at the time of its formation. It is defined as φ(log M) = dN / dlog M ∝ M ^-Γ, where M is the mass of a star and N is the number of stars in a logarithmic mass interval. The value of the slope found by Salpeter (1955) for → low-mass and → intermediate-mass stars in the → solar neighborhood is Γ = 1.35. The IMF can be expressed also in linear mass units: χ(M) = dN / DM ∝ M ^-α. Note that χ(M) = (1 / M lm 10) φ(log M), and α = Γ + 1. In this formalism the Salpeter slope is α = 2.35. There is a third way for representing the IMF, in which the exponent is x = -α.
The IMF is not a single power law over all masses, from → brown dwarfs to → very massive stars (Kroupa, 2002, Science 295, 82). Different slopes have been found for different mass segments, as follows: α = 1.3 for 0.08 ≤ M_solar < 0.5; α = 2.3 for 0.5 ≤ M_solar < 1; α = 2.3 for 1 ≤ M_solar.
The IMF at low masses can be fitted by a → lognormal distribution (See Bastian et al., 2010, ARAA 48, 339 and references therein). See also → canonical IMF.

See also: → initial; → mass; → function.

The value of the phase corresponding to the origin of time. Same as the → epoch angle.

See also: → initial; → phase; → angle.

An instant of infinite density, infinite pressure, and infinite temperature where the equations of general relativity break down, if the standard Big Bang theory is extrapolated all the way back to time zero.
→ singularity.

See also: → initial; → singularity.

1. The first step or action of a matter; commencing move.
   

   2. An introductory act or step; leading action.

Etymology (EN): From Fr. initiative, from L. initiatus, p.p. of initiare “to begin,” from initium “a beginning,” → initial.

Etymology (PE): Bongâm, from bon “basis; root; foundation; bottom,” → fundamental, + gâm “step, pace,” → egress.

1. General: To force into a passage, cavity, or tissue.
   

2. Math.: Verbal form related to → injection.

Etymology (EN): From L. injectus, p.p. of injicere “to throw in or on,” from → in- “in” + -icere, combining form of jacere “to throw.”

Etymology (PE): Daršândan, from darešândan, from dar-, → in, +
ešândan, → eject.

1. General: The act of injecting. Something that is injected.
   

2. Physics: The process of introducing electrons or holes into a semiconductor so that their total number exceeds the number present at thermal equilibrium. → hole injection.
   

3. Math.: A one-to-one → mapping. In other words, a mapping from a → set A onto a set B such that for every element in B there is at the most an element in A.

See also: Verbal noun of → inject.

Of or pertaining to a → injection.

See also: Adj. related to → injection.

A small arm of the sea, a lake, or a river.

Etymology (EN): From → in- + let “to allow,” M.E. leten, O.E. lætan; cognate with Du. laten, Ger. lassen, akin to Gk. ledein “to be weary”, L. lassus “tired.”

Etymology (PE): Šâxâbé, literally “branch of water,” from šâx, → branch, + âb “sea, river, → water” + noun suffix -é.

Situated within or farther within; interior.

Etymology (EN): From O.E. inra, comparative of inne (adv.) “inside;” → in-.