An Etymological Dictionary of Astronomy and Astrophysics

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.