An Etymological Dictionary of Astronomy and Astrophysics

A luminous, hot, blue star whose spectrum is dominated by the lines of hydrogen, atomic helium, and ionized helium; also known as O-type star. This is the earliest → spectral type and the only → main sequence star in which ionized helium is present. The → effective temperatures
of these stars range from about 30,000 K to 50,000 K, their luminosities from 50,000 to 1,000,000 times that of → solar luminosity, and their masses from about 20 to 100 → solar masses. The hottest O-type stars display high ionization emission features such as N III and He II,
→ Of star. They are divided into subtypes O2, the hottest, to O9.7, the coldest. O-type stars are relatively rare, for each star of 100 solar masses there are 10⁶ stars of solar mass. They are relatively short-lived since
they spend only a few million years on the main sequence. The brightest O-type star in the sky visible with naked eye is → Alnitak. For prominent Galactic O stars see → HD 93129.

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

The circular configuration of magnetic field lines around a → magnetic null point. See also → X-point.

See also: O, the round letter of alphabet; → point.

Same as → O star.

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

A loosely bound grouping of O and B stars that typically stretches up to several hundred → light-years and may contain between a dozen and several hundred → O stars
and → B stars. The members of an OB association are young and of roughly the same age. OB associations dissipate in a few tens of millions of years.

See also: O and B, from spectral types; → association.

A collective designation for massive O and B stars.

See also: → O star; → B star.

A hot star belonging to one of the two groups of → EHB stars or → pre-WD stars.

See also: → OB star; → subdwarf.

A → carbon (C)-rich → O-type or → B-type star.

See also: → O star; → B star; → carbon; → star.

The symbol ÷ used to indicate → division. This symbol was first used as a → division sign by Johann Rahn (or Rhonius) (1622-1676) in 1659 in Teutsche Algebra.

Etymology (EN): From Gk. obelus “sharpened stick, spit, pointed pillar,” related to obelisk, originally used in ancient manuscripts to mark passages that were suspected of being corrupted, doubtful, or spurious.

Etymology (PE): → division sign.

The outermost of Uranus’ large satellites and the second largest. It has a diameter of 523 km and orbits 583,420 km from its planet.
Compared to Uranus’ moons Ariel, Titania, and Miranda, Oberon is heavily cratered.
Like all of Uranus’ large moons, Oberon is composed of roughly half ice and half rock. Oberon was discovered by Herschel in 1787.

See also: Oberon is the King of the Fairies and husband of Titania in Shakespeare’s Midsummer-Night’s Dream.

1. General: Anything that is visible or tangible.
   

2. Physics: A collection of masses, taken to be one.
   

3. Optics: The collection of points that acts as a source of light rays for an optical system.
   

4. Astro.: A general designation for material bodies studied in this science.
   

5. Math.: Anything that could be formally defined, and with which one may do deductive reasoning and mathematical proofs. → mathematical object
   

6. Computer science: A free standing chunk of code that defines the properties of some thing.
   

7. Grammar: In English grammar, a noun, noun phrase, or pronoun that is affected by the action of a verb (a direct object or an indirect object) or that completes the meaning of a preposition (the object of a preposition).
   

8. To raise or state an argument against.

Etymology (EN): From M.L. objectum “thing thrown down or put before” (the mind or sight), neutral of objectus, p.p. of obicere “to present, oppose, cast in the way of,” from ob “against” + jacere “to throw,” from PIE base *ye- “to do” (cf. Gk. iemi, ienai “to send, throw,” Hitt. ijami “I make”).

Etymology (PE): Barâxt “thing drawn against, before” from bar- + âxt. The prefix bar- “on; upon; against; before; at; in,” from 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”.
The second component âxt, contraction of âxté, p.p. of âxtan, variants âhixtan, âhiz- “to draw (a sword),” âhanjidan “to draw up, pull, extract,” Mid.Pers. âhixtan, âhanjitan “to draw out, pull up, extract,” Av. θanj- “to draw, pull, drive;” Proto-Iranian
*θanj-.
Kondâr, literally “action receiver,” from kon- present stem of kardan “to do, make,” → -or, used as action noun (like xor, xâb, foruš),

• dâr agent noun from dâštan “to receive” (Dehxodâ), originally “to have, hold, possess,” → property.

The → lens or lenses in the → object end of the body tube of a → microscope, by means of which the rays coming from the object examined are brought to a focus. Same as → objective.

An old term for the → objective lens of a → refracting telescope.

See also: → object; → glass.

Any language described by a → metalanguage. For example, the sentence “In Persian, the word setâré means “star” " is part of a metalanguage (here, English), and the language described (namely Persian) is an object language. Metalanguage and object language may be identical.

See also: → object; → language.

In an → optical system, the space between the object being viewed and the → entrance pupil. See also → image space.

See also: → object; → space.

In computer science a programming technique that uses → objects and their interactions to design applications and programs.

Etymology (EN): → object; oriented, p.p. from the verb of → orientation; → programming.

Etymology (PE): Barnâme-sâzi, → programming; barâxt,
→ object; gerâ agent adj. of gerâyidan “to incline toward; to intend; to make for.”
The stem gerâ may be a variant of Mod.Pers. kil “bent, inclined” (k/g and l/r interchanges),
from PIE base *klei- “to lean, incline,” cognate with L. clinare “to bend” (E. declination, inclination, etc.), Gk. klinein “to cause to slope, slant, incline,” Skt. sri- “to lean,” O.Pers. θray-, Av. sray- “to lean,” P.Gmc. *khlinen (Ger. lehnen, E. lean).

1. The act of objecting.
   

   2. A statement presented in opposition.

See also: Verbal noun of → object.

1a) Of or pertaining to something that can exist independent of thought or perception as part of reality. Opposite of → subjective.

1b) Free of any bias or prejudice caused by personal feelings.

2. The → lens or combination of lenses nearest the → object in an optical instrument ( → telescope, → microscope, → camera).
   

3. The → primary mirror of a → reflecting telescope.
   

4. Grammar: Relating to nouns, noun phrases, or pronouns that are the objects of verbs or prepositions.

See also: Adjective of → object.

A diffraction grating placed over the aperture of a telescope in order to produce spectra of all the objects in the field of view.

See also: → objective; → grating.

The lens or system of lenses in a → telescope or → microscope that magnify an object and project a larger → image. The object lens is nearest the object being viewed

See also: → objective; → lens.

A dispersing prism placed in front of a telescope
objective to produce spectra of all luminous objects in the field of view.

See also: → objective; → prism.

A probability determined by the long-term frequency of an event; in contrast to subjective probability.

See also: → objective; → probability.

1. The state or quality of being → objective. Opposite of → subjectivity.
   

2. Ability to view things objectively.
   

3. External reality.

See also: → objective + → -ity.

Adjective meaning flattened, as → oblate spheroid. Opposed to → prolate.

Etymology (EN): From L. ob- “toward” + latus, abstracted from its opposite, prolatus “lengthened.”

Etymology (PE): Pax “oblate,” of unknown etymology.

An ellipsoid produced by rotating an ellipse through 360° about its minor axis. Compare with → prolate spheroid.

See also: → oblate; → spheroid.

The property possessed by a round body that is flattened at the poles.

See also: Noun from → oblate.

1a) To bind or oblige morally or legally.

1b) To pledge, commit, or bind (funds, property, etc.) to meet an obligation.

2a) Morally or legally bound; obliged; constrained.

2b) Necessary; essential (Dictionary.com).

See also: Back-formation from → obligation.

1. Something by which a person is bound or obliged to do certain things, and which arises out of a sense of duty or results from custom, law, etc.
   

2. Something that is done or is to be done for such reasons (Dictionary.com).

Etymology (EN): M.E. obligacioun, from O.Fr. obligacion “obligation, duty, responsibility,” from L. obligationem “an engaging or pledging,” literally “a binding,” noun of action from p.p. stem of obligare “to bind, bind up, bandage,” from → ob- “to” + ligare “to bind,” from PIE root *leig- “to bind.”

Etymology (PE): Fariz, from Mid.Pers. frêz “obligation; duty.”

Required as a matter of obligation; mandatory; incumbent; compulsory.

Etymology (EN): M.E., from L.L. obligatorius “binding,” → obligate, + -tory, from L. -torius, from → -tor + -ius.

Etymology (PE): Farizgun, from fariz, → duty, + -gun “manner, fashion.”

Neither perpendicular nor horizontal; having the axis not perpendicular to the base; slanting; sloping.

Etymology (EN): From M.Fr. oblique, from L. obliquus “slanting, sidelong, indirect,” from ob “against” + root of licinus “(of an ox whose corn tips are) bent upward,” from PIE base *lei- “to bend, be movable.”

Etymology (PE): Yekvar, literally “slantingn to a side,” from yek-, → one,

• var “side,” variant
  bar “side; breadth; breast” (Mid.Pers. var “breast;” Av. vouru “wide, broad, extended” (vourucašāni- “looking far”), related to varah- “breast;” cf. Skt. urú- “wide, broad,” úras- “breast;” Gk. eurus “wide, broad;” PIE base uer-, ueru-s“wide, broad”).

A stellar model in which the rotation axis is tilted relative to the magnetic dipole axis. As the star rotates, observable quantities (e.g. the line-of-sight component of the magnetic field, stellar brightness, emission lines) are modulated according to the rotational period. Such a model was first put forward by D. Stibbs (1950, MNRAS, 110, 395).

See also: → oblique; → rotator; → model.

A → shock wave that is inclined to the flow direction. Depending on the shape of the object and the speed of the → flow, the shock wave may be inclined to the flow direction.

See also: → oblique; → shock.

The celestial sphere when the circles parallel to the equator are cut
obliquely by the horizon plane, which divides them into two unequal parts.
In other words, the sphere when its axis is oblique to the horizon of the place.

See also: → oblique; → sphere.

1. General: The quality or state of being oblique.
   

2. The → angle that an object’s → rotational axis makes with its → orbital axis. In other words, the angle between its → equatorial plane and → orbital plane. Same as → axial tilt.

See also: State noun of → oblique.

The angle between the Earth’s → equatorial plane and the → ecliptic. Its present value is 23°26'13.5’' (or 23.43708 degrees). The effects of → precession and → nutation cause it to change between extreme values of 22.1 and 24.5 degrees over a period of approximately 41,000 years. It is currently decreasing.

See also: → obliquity; → ecliptic.

Elongated, usually from the square or circular form.

Etymology (EN): Late M.E. oblonge, from L. oblongus “rather long,” from ob- a prefix meaning “toward, to, on, over, against”

• longus, → long.

Etymology (PE): Derâzak, from derâz “long,” → length + -ak suffix.

A → nitrogen (N)-rich → O-type or → B-type star.

See also: → O star; → B star; → nitrogen; → star.

1. Opposition to the spread of knowledge; a policy of withholding knowledge from the general public.
   

   2. A style (as in literature or art) characterized by deliberate vagueness or abstruseness (Merriam-Webster.com).

See also: From Fr. obscurantisme (1819), from obscurant, from obscure, from L. obscurus “dark, dusky,” figuratively “unknown; unintelligible,” from ob- “over” + -scurus “covered,” from PIE *(s)keu- “to cover, conceal;” → -ism.

1. The act of obscuring.
   

2. The state of being obscured.

See also: → obscure; → -tion.

1a) Lacking in light or illumination; dark; dim; murky.

1b) Indistinct to the sight or any other sense.

1c) Not clear to the understanding.

2. To make dark, dim, indistinct, etc.

Etymology (EN): From O.Fr. obscur “dark, dim, not clear,” from L. obscurus “covered over, dark, obscure,” from ob “over” + -scurus “covered,” from PIE *(s)keu- “to cover, conceal;” from which derives also the term → sky.

Etymology (PE): Tiré, from Mid.Pers. târag “dark, turbid,” related to târik “dark,” Mid.Pers. târig “dark,” târ “darkness,” Av. taθra- “darkness,” taθrya- “dark,” cf. Skt. támisrâ- “darkness, dark night,” L. tenebrae “darkness,” Hittite taš(u)uant- “blind,” O.H.G. demar “twilight.”
Tiré kardan compound verb with kardan “to do,” → -ize.

A structure of dust and gas postulated to surround the central → black hole in an → active galactic nucleus (AGN). The presence of an obscuring torus allows the unification of the two main types of AGNs containing a → broad-line region (Type I) and a → narrow-line region (Type II), respectively. In this unified model, the two types represent the same sort of object, the appearance of which depends on the viewer’s → line of sight.
The best evidence for this model comes from spectropolarimetry observations of some type II AGNs in which broad → emission lines are seen in → polarized light, as would happen if the broad-line region truly were hidden, and the light were being reflected off the torus and into the viewer’s line of sight.

See also: → obscure; → torus.

The state or quality of being obscure.

See also: → obscure + → -ity.

1. General: The character of something that can be observed.
   

2. Astro.: The possibility that an object be observable at a particular time.
   

3. Control theory: A measure for how well internal states of a system can be inferred by knowledge of its external outputs.

See also: From → observable + → -ity.

1. Capable of being or liable to be observed.
   

2. A quantity that can be measured.
   

3. Quantum mechanics: A → real dynamical variable whose → eigenstates form a → complete set.

See also: From → observe + → -able.

The extent of the Universe that we can see with the aid of the largest telescopes. Its ultimate boundary is determined by the → cosmic horizon size.

See also: → observable; → universe.

1. An act or instance of following a custom, rule, or law.
   

2. A customary practice, rite, or ceremony; a rule governing members of a religious order.
   

3. An act or instance of watching (Merriam-Webster.com).

Etymology (EN): → observe; → -ance.

Etymology (PE): Nepâhdâri, on the model of negâhdâri “preservation, protection,” from nepâhdâr, → observant, + -i.

1. Paying strict attention.
   

2. Careful in observing (Merriam-Webster.com).

Etymology (EN): → observe + -ant, a suffix forming adjective, → -ance.

Etymology (PE): Nepâhdâr, from nepâh present stem of nepâhidan, → observe, + dâr preset stem and agent noun of dâštan, dâridan “to have, hold, maintain, possess,” → protected band.

1. Act or instance of observing; → observe.
   

2. Careful watching of an astronomical object or event using appropriate instruments, usually for collecting data.

See also: Verbal noun of → observe.

Pertaining to, or founded on observation, especially based on observation rather than theory.

See also: Adj. of → observation.

That part of astrophysics that is mainly concerned with the collection of observational data, in comparison with theoretical astrophysics

See also: → observational; → astrophysics.

An error in observation arising from systematically favoring brighter or weaker objects or some particular object morphologies; e.g. → Malmquist bias.

See also: → observational; → bias.

The application of observational data to the study of the Universe as a whole.

See also: → observational; → cosmology.

A feature appearing in an observation, which is not intrinsic to the object observed, but is due to the inappropriate method used (e.g. limited angular resolution).

See also: → observational; → effect.

The difference between a measured value of quantity and its true value.

See also: → observational; → error.

A place or building equipped for making observations of astronomical, meteorological, or other natural phenomena, especially a place provided
with a telescope for observing astronomical objects.

Etymology (EN): From Fr. observatoire, from
L. observa(re), → observe, + -toire, from L. -torius, from -tor a suffix forming agent nouns + -ius adj. suffix.

Etymology (PE): Nepâhešgâh, from nepâheš, → observation,

• -gâh suffix of place (O.Pers. gāθu-, Av. gātav-, gātu- “place, throne, spot” (Skt. gátu- “going, motion; free space for moving; place of abode,” PIE *gwem- “to go, come”).

To watch carefully or note for a scientific or special purpose, e.g. to observe a star (astronomy), to observe the behavior of a patient (medicine, psychology), an animal (ethology, zoology), social groups (sociology), etc.

Etymology (EN): From O.Fr. observer, from L. observare “watch over, look to, attend to, guard,” from ob “over” + servare “to watch, keep safe,” from PIE base *ser- “to protect;” cf. Av. har- “to guard, observe, pay attention to,” haraiti “guards, keeps,” harətar- “protector, watcher,” harəθra- “guarding, protection,” hāra- “watched, guarded,” Mod.Pers. zinhâr “beware!, mind!,” Gk. heros “protector, hero.”

Etymology (PE): Note 1: Observation is the most important basis of empirical sciences. All theories rely on observation, and must finally be supported by observational evidence. Persian, in contrast to European languages, lacks a distinct term that recognizes observation as a conceptual premise of sciences. In astronomy the Ar. rasad (رصد) is currently used, while in classical astronomical texts the Pers. pâyidan (پاییدن) is used mainly by prominent figures like Biruni and Tusi. However, the term rasad is never used for cases outside astronomy. The situation for other sciences is not clear. Often one resorts to the Ar. loanword mošâhedé
“to look at each other,” which lacks ability to form derivatives. Therefore, here we suggest a term that can be used in all fields of knowledge.

Nepâhian, from ne- + pâhidan. The prefix ne- “down; into” (as in negâh “look, glance,” negaristan “to view, look at,” nešastan “to sit down,” nemudan “to display”) from O.Pers./Av. ni- “down, into;” cf.
Skt. ni “down,” nitaram “downward,” Gk. neiothen “from below,” cf. E. nether, O.E. niþera, neoþera “down, downward, below, beneath,” from P.Gmc. *nitheraz, Du. neder, Ger. nieder;
PIE *ni- “down, below.”
The second component pâhidan, variants pâyidan, pâsidan “to watch, observe, look steadily;” Mid.Pers. pây- “to protect, guard;” Sogdian p’y “to observe, protect, watch over;” O.Pers. pā- “to protect,” pāta- “protected;” Av. pā- “to protect,” pāti “guards,” nipā(y)- (with ni-) “to watch, observe, guard,” nipātar- “protector, watcher,” nipāθri- “protectress;”
cf. Skt. pā- “to protect, keep,” tanû.pā- “protecting the body,” paś.pā- “shepherd;” Gk. poma “lid, cover,” poimen “shepherd;” L. pascere “to put out to graze,” pastor “shepherd;” Lith. piemuo “shepherd;” PIE base *pā- “to protect, feed.”

Note 2: Since pây-, pâyidan is used for → conserve, we adopt the variant pâh-, pâhidan, which is extant in Lâri and
Gerâši dialects; cf. Gilaki pâstan, pâssan “to guard, pay attention.” In fact the interchange of phonemes h, i and s is not rare in Indo-European languages.

Note 3: Examples of pâyidan “to observe” in astronomy, from Biruni’s al-Tafhim (written c. 1029 A.D.)
p. 313: و بپای ارتفاع آ فتاب را.
p. 64: و بپای تا بدایره اندر آید.

Note 4: As indicated above, nepâh- exists in Av. as nipāy- “to watch, protect.”

Pertaining to a value which has been measured, in contrast to one which is computed.

See also: Past participle of → observe.

1. Someone or something that observes.
   

2. Idealized person or piece of equipment, often hypothetical, that measures relevant properties of a physical system.

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

A place located either at the top of the tube of a large telescope
from where one observes or at the back of the tube where instruments are
attached to the → Cassegrain focus.

See also: → observer; → cage.

A written report provided by a visiting astronomer to the observatory describing how his/her observing run has been carried out.

See also: Observing, noun of → observe; → report.

A period of telescope time for observations allocated to a science project.

See also: Observing, noun of → observe; → run.

The long opening in the dome of a telescope through which observation is conducted.

See also: Observing, noun of → observe; → slit.

The state, process, or condition of being or becoming obsolete or outdated and no longer used.

See also: → obsolete; → -escence.

Becoming obsolete; passing out of use.

See also: → obsolete; → -escent.

1a) No longer in general use; fallen into disuse.

1b) Biology: (of a living organism) Reduced to a redument, or completely lacking, as compared with a corresponding part in related organisms.

2. To make obsolete by replacing with something newer or better.

Etymology (EN): From L. obsoletus “grown old, worn-out,” p.p. of obsolescere “fall into disuse, be forgotten about, become tarnished,” which probably is from ob “away” + an expanded form of solere “to be used to, be accustomed.”

Etymology (PE): Pitâr, from Tabari pitâr “worn out,” specifically “rotten tree,” pitə-pât “anything worn out or useless,” from pit, variant of Pers. pud “worn out, decayed,” pusidan/pus- “to rot,” cf. Pashto puda, Wakhi pitk “rotten, foul,” Kurd. (Kurm.) puc “rotten, useless;” Av. pu- “to stink, rot,” akin to Gk. puos, L. pus “pus.”

An angle whose measure is greater than 90° and less than 180°.

Etymology (EN): Obtuse, from M.Fr. obtus (fem. obtuse), from L. obtusus “blunted, dull,” p.p. of obtundere “to beat against, make dull,” from ob “against” + tundere “to beat,” from PIE *(s)tud- “to beat, strike, push, thrust;” → angle.

Etymology (PE): Zâviyé, → angle; bâz “open,” from Mid.Pers. abâz-, apâc-, O.Pers. apa- [pref.] “away, from;” Av. apa- [pref.] “away, from,”
apaš [adv.] “toward the back;” cf. Skt. ápāñc “situated behind.”

1. A particular time, especially as marked by certain circumstances or occurrences.
   

2. A special or important time, event, ceremony, celebration, etc.
   

3. A convenient or favorable time, → opportunity, or juncture (Dictionary.com).

Etymology (EN): M.E. occasioun from O.Fr. ochaison, ocasion “cause, reason, pretext; opportunity,” from L. occasionem (nominative occasio) “appropriate time,” from occasum, occasus “fall; sunset,” p.p. of occidere “fall down, go down,” from ob “down, away” + cadere “to fall,” → case.

Etymology (PE): Âkatan, from prefix â- + katan “to fall,” cf. Laki: katen “to fall,” kat “he/she fell,” beko! “fall!” (an insult);
katyâ “fallen;” Lori: kat “event, error;” Kurd. (Soriani): kawtin “to fall, befall,” kett “fallen;” Kurd. (Kurmanji): da.ketin “to fall down;” Lârestâni: kata “to fall;” Garkuyeyi: darkat, varkat “he/she fell (sudden death);” Gilaki (Langarud, Tâleš): katan “to fall,” bakatam “I fell,” dakatan “to fall (in a marsh, in a pit),” vakatan “to fall from tiredness, be exhausted,” fakatan “to fall from (i.e., lose) reputation;” Tabari: dakətə “fallen,” dakətən “to crash down,” dakət.gu “stray cow;” Proto-Iranian *kat- “to fall;” cf.
L. cadere “to fall,” → case, Pers. kâté.

1. Occurring or appearing at irregular or infrequent intervals; occurring now and then.
   

2. Intended for supplementary use when needed.
   

3. Pertaining to, arising out of, or intended for the occasion (Dictionary.com).

See also: → occasion; → -al.

An → impact crater on the → dwarf planet  → Ceres. It has a diameter of about 90 km and a depth of about 4 km.

See also: Named after the Roman agriculture deity of harrowing, a method of leveling soil.

1. To block or stop up (a passage or opening); obstruct. To prevent the passage of.
   

   2. Chem.: To entrap a substance (vapor, gas, liquid, solid) within another (by absorption).

Etymology (EN): From L. occludere “shut up, close up,” from ob “in front of, against” + claudere “to shut, close,” → include.

Etymology (PE): Barkolândan, from bar- “on; upon; against; before; at,” → object, + kolândan “to shut,” → include.

1. The process of occluding. Something that occludes.
   

   2. Phonetics: Closure at some point in the vocal tract that blocks the flow of air in the production of an oral or nasal stop (TheFreeDictionary.com).

   3. Chem.: An absorption process by which a substance adheres strongly to another.

See also: → occlude; → -tion.

1. Occluding or tending to occlude.
   

   2. Phonetics: An oral or nasal stop. → occlusive consonant.

See also: → occlude; → -ive.

A → phoneme that is produced following the obstruction of air flow in the → pharynx. The sound arises during the exhalation when the air passage is no longer blocked. English occlusive consonants are [p],[b],[t],[d],[k], and [g]. Same as → stop consonant.

See also: → occlude; → -ive.

The blocking of light from an astronomical object, such as a star,
by another object, such as the Moon or an asteroid, that passes in front of it. Also, the period of time for which this blocking takes place. A → solar eclipse is a particular case of occultation. Determining the moment of stellar occultation by the Moon is used to improve our knowledge of the lunar orbit. Moreover, the occultation of stars by asteroids or planetary satellites provide important data for better measuring the sizes of the occulting bodies. See also → grazing occultation.

Etymology (EN): M.E. from L. occultation- “a hiding,” from occultat(us), p.p. of occultare “to conceal, keep something hidden,” frequentative of occulere “to cover over, conceal,” from ob “over” + a verb related to celare “to hide,” from PIE base *kel- “to conceal.”

Etymology (PE): Forupušân, 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”) + pušân p.pr. of pušândan, accusative of pôšidan, pôš- “to cover; to wear” (related to pust “skin, hide;” Mid.Pers. pôst; O.Pers. pavastā- “thin clay envelope used to protect unbaked clay tablets;” Skt. pavásta- “cover,” Proto-Indo-Iranian *pauastā- “cloth”).

The act of occupying; the state of being occupied.

See also: Verbal noun of → occupy.

The energy level adopted by a particle, atom, or molecule under quantum mechanical laws.

See also: Occupied, p.p. of → occupy;
→ level.

To take or fill up (space, time); to take possession and control of a place.

Etymology (EN): From O.Fr. occuper, from L. occupare “take over, seize, possess, occupy,” from ob “over” + intensive form of capere “to grasp, seize, take;” PIE base *kap- “to grasp” (cf. Skt. kapati “two handfuls;” Gk. kaptein “to swallow,” O.Ir. cacht “servant-girl,” literally “captive;” Goth. haban “have, hold;”
O.E. habban “to have, hold,” probably Mod.Pers. qâp-, qâpidan “to seize;”
Av. haf-, hap- “to keep, observe.”

Etymology (PE): Hagidan, from dialectal Pers.: Šahmirzâdi hâgetan, Saraxsi hagiton, Tabari hâytan, haytan,
Semnâni hâtan, Gilaki hâgitan, Lâsgardi agiton, degiton “to occupy;”
maybe related to Av. haz- “to occupy;” Skt. sah- “to overcome, be victorious, prevail,” saha- “victory,” sahate “overcomes, masters;” Gk. ekhein “to have, hold;” PIE base *segh- “to have, to hold.”

1. To happen; take place; come to pass.
   

2. To be found or be present; exist.

Etymology (EN): M.Fr. occurrer “to happen unexpectedly” or directly from L. occurrere “run to meet, run against, present itself,” from ob “against, toward” + currere “to run,” → current.

Etymology (PE): Roxdâdan, literally “to appear,” from rox “appearance; aspect; face,” variant ruy “face, surface” + dâdan “to give,” → event.

1. The action, fact, or instance of occurring.
   

2. Something that happens; event; incident (Dictionary.com).

3. Logic: → bound occurrence; → free occurrence.

See also: Verbal noun of → occur.

The intercommunicating body of salt water occupying the depressions of the Earth’s surface, or one of its major primary subdivisions, bounded by the continents, or the equator, and other imaginary lines. A sea is subdivision of an ocean.

the vast body of salt water that covers almost three fourths of the earth’s surface.

Etymology (EN): M.E. ocean(e), from O.Fr. occean, from L. oceanus, from Gk. okeanos “the great river or sea surrounding the disk of the Earth,
as opposed to the Mediterranean,” of unknown origin. Personified as Oceanus, son of Uranus and Gaia and husband of Tethys.

Etymology (PE): Oqyânus, from Ar., ultimately from Gk., as above.

A hypothetical → exoplanet covered by a water envelope. The presence of such a planet stems from the implicit assumption of → Habitable Zone temperatures and a liquid water surface.

See also: → ocean; → planet.

Of, living in, or produced by the ocean.

See also: → ocean; → -ic.

That part of the → Earth’s crust underling most of the Earth’s surface which is covered by the oceans. It has a remarkably uniform composition (mostly ~ 49% SiO₂) and thickness (mostly ~ 7 km). The ocean floor is the most dynamic part of the Earth’s surface. As a result, no part of the oceanic crust existing today is more than 200 million years old, which is less than 5% of the age of the Earth itself. New oceanic crust is constantly being generated from the → upper mantle by sea-floor spreading at → mid-ocean ridges, while other parts of the oceanic crust are being recycled back into the mantle at subduction zones.

See also: → oceanic; → crust.

Any section of the narrow, continuous submarine mountain chain through all the world’s oceans. The oceanic ridge constitutes the most extensive mountain ridge on Earth, more than 65,000 km. Perhaps the best-known part of the ridge system is the → Mid-Atlantic Ridge.

See also: → oceanic; → ridge.

The study of the ocean, embracing and integrating all knowledge pertaining to the ocean’s physical boundaries, the chemistry and physics of sea water, and marine biology.

Etymology (EN): → ocean + → -graphy.

Etymology (PE): Oqyânus→ ocean + -šenâsi→ -logy.

Any of various natural clays rich in ferric oxides (Fe₂O₃) and containing silica and alumina. Used as a red (→ hematite) or yellow (→ limonite) pigment.

Etymology (EN): M.E. oker, O.Fr. ocre, from L. ochra, from Gk. okhra,
from okhros “pale yellow.”

Etymology (PE): Oxrâ, loan from Gk.

The notion that any hypothesis should be stripped of all unnecessary assumptions. If two hypotheses fit the observations equally well, the one that makes the fewest assumptions should be chosen.

Etymology (EN): The doctrine was formulated by William of Ockham (c.1288-c.1347),
an English Franciscan friar and scholastic philosopher. Razor from O.Fr. rasour “a razor,” from raser “to scrape, shave.”

Etymology (PE): Ostaré “razor,” from sotordan “to shave, erase, remove;”
Mid.Pers. ôstarak “removed, shaved,” ôstârišn “wiping, shaving;”
cf. Khotanese ustar- “to remove,” Sogdian (prefixed *pari-) prtr- “to wipe off, dry up,”
from Proto-Iranian *us-tar- “to remove, wipe off.”

A prefix meaning eight.

Etymology (EN): From L. octo, Gk. okto, cognate with Pers. hašt, as below.

Skt. asta, Goth. ahtau, O.E. eahta (see eight).

Etymology (PE): Hašt “eight,” from Mid.Pers. hašt, O.Pers.*aštahva- “eighth;” Av. ašta; cf. Skt. astā; Ossetic ast; (Buddhist) Sogdian ‘št; Gk. okto, L. octo (Fr. huit; Sp. ocho);
P.Gmc. *akhto(u) (O.E. eahta, æhta, E. eight, O.N. atta, Ger. acht, Goth. ahtau); PIE base *oktô(u).
Octa-, oct, loaned Gk. forms, as above.

A group of eight units or figures.
Chemistry: An element, atom, or group having a valence of eight.

See also: From Gk. oktad- (stem oktás) “group of eight,” from okt-→ oct- + -ad a prefix denoting a group or unit comprising a certain number, sometimes of years (e.g. dyad; triad).

A polygon having eight angles and eight sides.

Etymology (EN): From L. octagonos, from Gk. oktagononos “eight-angled,” from okta-, → octa-, oct- “eight,” + gonia “angle,” related to gony “knee,”
L. genu “knee,” cuneus “a wedge;” Av. žnu- “knee;” Mod.Pers. zânu “knee,” Skt. janu- “knee,” kona- “angle, corner;” PIE base *g(e)neu-, see below.

Etymology (PE): Haštbar “eight-sided,” from hašt “eight,” → octa-, oct- + bar “side; breadth; breast” (Mid.Pers. var “breast;” Av. vouru “wide, broad, extended” (vourucašāni- “looking far”), related to varah- “breast;” cf. Skt. urú- “wide, broad,” úras- “breast;” Gk. eurus “wide, broad;” PIE base uer-, ueru-s“wide, broad”); haštguš, from hašt, → octa-, oct-, + guš “corner, angle,” Mid.Pers. gošak “corner.”

A geometric solid with eight sides.

See also: → octa-, oct-; → -hedron.

The Octant. A faint and obscure constellation, at 21h right ascension, 80° south declination, containing the south celestial pole. Its star Sigma Octantis is the closest naked-eye star to the pole, but it is so faint (magnitude 5.47) that it is practically useless as a polar star for navigation purposes. Abbreviation: Oct; Genitive: Octantis. It was introduced by the French astronomer Nicolas Louis de Lacaille (1713-1762).

Etymology (EN): From L.L. octans “eighth part of a circle,” from → octa-, oct- + -ans, as in quadrans; → quadrant.

Etymology (PE): Haštakân, → octant.

1. A portion of a circle cut off by an arc and two radii at 45°, one-eighth of the area of a circle.
   

2. A instrument incorporating a telescope, the predecessor of the → sextant, used to measure angular distances, mainly in nautical astronomy. It spanned an angular range of 45°.

Etymology (EN): From L.L. octans “eighth part of a circle,” from → octa-, oct- + -ans, as in quadrans; → quadrant.

Etymology (PE): Haštakân, from haštak “one-eigth,” from hašt “eight” (Mid.Pers. hašt, O.Pers.*aštahva- “eighth;” Av. ašta; cf. Skt. astā; Ossetic ast; (Buddhist) Sogdian ‘št; Gk. okto, L. octo (Fr. huit; Sp. ocho);
P.Gmc. *akhto(u) (O.E. eahta, æhta, E. eight, O.N. atta, Ger. acht, Goth. ahtau); PIE base *oktô(u)

• -ak, contraction of yak “one,” (Mid.Pers. êwak (Proto-Iranian *aiua-ka-); O.Pers. aiva- “one, alone;” Av. aēuua- “one, alone” (cf. Skt. éka- “one, alone, single;” Gk. oios “alone, lonely;” L. unus “one;” E. one) + -ân nuance suffix.

The interval between two musical notes, the fundamental components of which have frequencies in the ratio two to one.

Etymology (EN): M.E., from O.Fr. otaves, from L. octava feminine of octavus, from
→ octa-, oct- + -avus adj. suffix.

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

General: A group or series of eight.
Chemistry: A stable group of eight electrons which constitutes the outer electron shell of an atom of an inert gas (except helium whose only electron shell contains two electrons).

Etymology (EN): From → oct-, octa- + -et, as in duet.

Etymology (PE): Haštâyé, from haštâ “eightfold” + (y)é nuance suffix, as in dotâyé, → doublet.

Eightfold; eight times as great.

Etymology (EN): L. octuplus, from octu- variant (before labials) of → oct- octa- + -plus “fold,” from base of plicare “to fold, twist.”

Etymology (PE): Haštâyi, from hašt “eight,” → oct- octa- + -tâyi, from tâ “fold, plait, ply; piece, part,” also a multiplicative suffix; Mid.Pers. tâg “piece, part.”

A → multipole consisting of eight point charges. Octupole moments are much smaller than → quadrupole moments and very much smaller than → dipole moment.

See also: → oct-; → pole.

1. Of, pertaining to, or for the eyes.
   

2. Optics: An alternative term for → eyepiece.

Etymology (EN): From L. ocularis “of the eyes,” from oculus “eye,” from PIE base *ok^w- “to see;” cf; Av. aši- “(both) eyes;” E. → eye.

Etymology (PE): Cašmi, related to cašm “eye” (Mid.Pers. cašm, Av. cašman- “eye,” ākas- “to look,” from prefix ā- + Proto-Iranian *kas- “to look, appear,” cf. Skt. cáksus- “seeing”); didgâni, related to didgân “eyes,” plural of didé “eye,” from didan “to see” (Mid.Pers. ditan “to see, regard, catch sight of, contemplate, experience;” O.Pers. dī- “to see;” Av. dā(y)- “to see,” didāti “sees;” cf.
Skt. dhī- “to perceive, think, ponder; thought, reflection, meditation,” dādhye; Gk. dedorka “have seen”).

Of a number, not divisible by two.
A function such as f(x) with the
property that f(-x) = - f(x) for any value of x; e.g. sin(x).

Etymology (EN): From M.E. odde, from O.N. oddi “odd (number).”

Etymology (PE): Tâq, related to tak “single, alone”, from Mid.Pers. tak, tâg “single, alone,” maybe related to tâi, tâ “unit, piece.”

Nucleus which contains an odd number of protons and an even number of neutrons.

See also: → odd; → even; → nucleus.

Nucleus which contains an odd number of both protons and neutrons.

See also: → odd; → nucleus.

The science of → wines. Same as enology.

Etymology (EN): From Gk. oeno-, from oinos cognate with → wine.

Etymology (PE): Bâdešenâsi, from bâdé, bâda “wine,” from Mid.Pers. bâtak “wine,” + -šenâsi, → -logy.

The unit of magnitude of magnetic field strength or magnetic intensity in c.g.s. units, i.e. the force in dynes which a unit magnetic pole would experience at any point in a magnetic field.

See also: In honor of Hans Christian Ørsted (1777-1851), the Danish physicist and philosopher, who was the first to notice the interaction of electric current and the magnetic needle (1819) thereby initiating the study of electromagnetism.

An → O star whose spectrum displays strong
N III 4634-4640-4642 emission and strong He II 4686 emission.
The N III lines are always much stronger than C III 4647-4650-4651 when the latter are present. Historically, Of stars were considered to belong to the peculiar category, hence the f notation (see below). In his thesis work, Walborn (1971, ApJS 23, 257) removed them from that category and established them as the normal O-type → supergiants. He also used the notation ((f)), (f), and f to describe the progression from strong He II 4686 absorption, through weakened/absent, to emission, respectively, correlated with increasing N III emission strength, subsequently showing that it is a luminosity sequence – the first such for stars earlier than O9.

See also: The reason for the Of designation is that the letters Oa-Oe were used in the original Harvard classification to denote various types of → Wolf-Rayet and OB spectra. Therefore Of was the next available when Plaskett and Pearce (1931, Pub. Dominion Ap. Obs 5, 99) wished to distinguish O-type spectra with selective emission in N III 4634-4640-4642 and He II 4686 (“selective” because other lines from the same ions appear in absorption); → star.

A → massive star spectrum whose principal defining characteristics is the presence of C III 4647, 4650, 4651 emission lines with strength comparable to that of N III 4634, 4640, 4642. This category was introduced by Walborn (1972) to describe two well-known peculiar stars, HD 108 and HD 148937. → Of star

See also: → Of star; the question mark was intended to denote doubt that these stars are normal Of supergiants; p for “peculiar.”

(adverb & preposition) From a place or position; at a distance in space or time. So as to be separated from support.

See also: M.E., from O.E. of “away, away from;” cf. Du. af “off, down,” Ger. ab “off, from, down;” PIE *apo- “off, away,” → apo-.

Away from a place.

Etymology (EN): → off; cognate with Av. and O.Pers. apā “away from, from,” as below.

Etymology (PE): Ap-, from apâ-, from Av. and O.Pers.
apā “away from, from;” cf. Skt. apa “away, off;” L. ab- “from, away;” Hittite appa; Gothic af-; Ger. ab-; E. of, as above; PIE base *apo- “off, away”

An → optical system in which the → optical axis of the → aperture is not coincident with the mechanical center of the aperture.

See also: → off; → axis; → optical; → system.

Of computers, operating independently of, or disconnected from, an associated computer.

See also: → off; → line.

An observation when the telescope is pointed away from the source in order to measure the sky background contribution.

See also: → off-; → source;
→ observation.

1. Cause to feel upset, annoyed, or resentful.
   

   2. Commit an illegal act; break a commonly accepted rule or principle (OxfordDictionaries.com).

Etymology (EN): M.E. offenden, from O.Fr. ofendre “transgress, antagonize,” and directly from L. offendere “to hit, strike against,” figuratively “to stumble, commit a fault, displease,” from assimilated form of ob “in front of against” + -fendere “to strike, push,” from PIE root *g^when- “to strike, kill;” cf. Av. -γna- “slaying,” → murder.

Etymology (PE): Âfandidan, from âfand “strife, war,” probably from Proto-Iranian *â-fanda-, from prefix *â- + *fanda-, from *fan- “to move;” cf. Yazghulami fin-/fud “to descend, come down,” fəndan- “to bring down;” Roshani sifan-, Bartangi sifân- “to rise;” Skt. phan- “to jump” (Cheung 2007).

A person who commits an illegal act; a person or thing that does something wrong or causes problems (OxfordDictionaries.com).

See also: → offend; → -er.

1. A breach of a law or rule; an illegal act.
   

   2. Annoyance or resentment brought about by a perceived insult to or disregard for oneself.
      

   3. The action of attacking someone or something (OxfordDictionaries.com).

See also: M.E. offence, offense, from O.Fr. ofense and directly from L. offensa “an offense, affront, crime,” literally “a striking against,” noun use of fem. p.p. of offendere, → offend.

1. Causing resentful displeasure; highly irritating, angering, or annoying.
   

   2. The position or attitude of aggression or attack (Dictionary.com).

See also: → offense; → -ive.

1. A shift in the pointing position of a telescope with respect to a reference position.
   

2. Another term for the → bias in → CCD detectors.
   

3. Surveying: A short distance measured perpendicularly from a main survey line.

Etymology (EN): → off- + → set.

Etymology (PE): Ap-, → off-;

• neh present stem of nehâdan “to place, put; to set” Mid.Pers. nihâtan;
  Av. ni- “down; below; into,” → ni-,
• dā- “to put; to establish; to give,” dadāiti “he gives;” cf. Skt. dadāti “he gives;” Gk. didomi “I give;” L. do “I give;” PIE base *do- “to give.”

Guiding an astronomical exposure on a star, when the object of interest is nearby, but invisible.

See also: → offset; → guiding.

A small class of evolved → massive stars showing spectral properties intermediate between those of → Of star and → WN Wolf-Rayet stars. Several of them have been found to possess non-spherical nitrogen-rich circumstellar nebulae. Ofpe/WN9 stars are considered to be transition objects between Of and W-R stars. This type of stars was first identified by Walborn (1982), who introduced the classification Ofpe/WN9, indicating that the stars could not be classified solely as Of stars, nor as WNL stars. Ofpe/WN9 stars have been found in the → Milky Way, the → Large Magellanic Cloud, → M31, and M33. Currently 10 Ofpe/WN9 stars are known in the LMC. Observational evidence suggests a close relationship between the class of → LBVs and the Ofpe/WN9 stars. A notable example is the prototype Ofpe/WN9 star R127 in the LMC that became an LBV on a time-scale of the order of a year. The possibility of such a relationship has been explored by Smith et al. (1994), who proposed that some LBVs show spectral morphologies that make them appear as an extension of the WN sequence toward later spectral types. Hence, they reclassified Ofpe/WN9 stars as WN10-11.

See also: Ofpe, from → Of star; p for “peculiar;” e refers to the presence of other emission lines in addition to the Of ones, mainly H and He I, although also Si III. This peculiar class in the LMC was first described by Walborn (1977, ApJ 215, 53), where
he called them “O Iafpe extr” based on the most similar Galactic objects known at that time. Subsequently Walborn (1983, ApJ 256, 452) and Bohannan & Walborn
(1989, PASP 101, 520) suggested an extension of the WN sequence and/or transition between Of and WN.
This nomenclature Ofpe/WN9 has been rather widely adopted. Later on, Smith et al. (1994) broke the Ofpe/WN9 and related Galactic types into WN10-11 subtypes to include this group of emission line stars.

Many times; frequently; in many cases.

Etymology (EN): M.E. oftin, from O.E. oft “often, frequently,” akin to cognates: O.Frisian ofta, Danish ofte, O.H.G. ofto, Ger. oft, of unknown origin.

Etymology (PE): Basvân, from bas “many, much,” → frequency, + -vân on the model of farâvân, → abundance.

1. A Hydroxyl radical formed by abstraction of a hydrogen atom from water.
   
2. An OH group within a molecule.

See also: From → hydro- + ox(y)- a combining form meaning “sharp, acute, pointed, acid,” used in the formation of compound words, from Gk, oxys “sharp, keen, acid” + -yl a suffix used in the names of chemical radicals, from Fr. -yle, from Gk. hyle “matter, substance.”

→ Calabash Nebula.

Emission or absorption lines on an electromagnetic spectrum generated by hydroxyl, → OH molecules. At present, four principal lines are known in the radio domain at frequencies of 1612, 1665, 1667, and 1720 MHz, or wavelengths of approximately 18 centimeters.

Etymology (EN): → OH; → line.

A → maser phenomenon created by → OH molecules with characteristic → OH lines. OH masers are detected toward a variety of astronomical environments, including massive star formation regions and evolved late-type stars.

Etymology (EN): → OH; → maser.

An astronomical source emitting microwave radiation characteristic of the hydroxyl OH molecule, especially one showing a maser effect. OH sources are found in molecular clouds in interstellar medium and in the cool envelopes of evolved stars.

See also: OH, chemical compound hydroxyl; → source.

An evolved Mira-type star which is associated with strong OH maser and strong infrared (IR) emission from the surrounding shell of warm gas and dust.

Etymology (EN): → OH; → infrared; → star.

A unit of electrical resistance equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals.

See also: Named after Georg Simon Ohm (1789-1854), the German physicist who discovered the law which bears his name.

1. For a → conductor at rest, the → voltage across the conductor is equal to the product of the current flowing through it and its → resistance. In other words, when such a conductor is subjected to an electric field E, the resulting → current density, J, is proportional to the electric field E: J = σE, where σ is the → conductivity, i.e. the reciprocal of → resistivity, ρ = 1/σ.
   

2. Ohm’s law for a moving conductor is expressed by:

J = σ(E + v x B), where v is the velocity and B
the → magnetic induction.

See also: → ohm; → law.

Of or relating to a system which obeys Ohm’s law.

See also: → ohm + → -ic.

An upper bound on the time scale on which the magnetic field of a system would decay in the absence of any other agent. It is expressed as: τ_μ = R² / μ, where R is the scale size of the system, η the magnetic diffusivity (η = 1 / μσ, where μ is the magnetic permeability and σ the electrical conductivity). For a star like the Sun, τ_μ  ≅ 10¹⁰ years, so a fossil magnetic field could survive for the star’s lifetime on the main sequence. For the Earth, τ_μ  ≅ 10⁴ years, so a → dynamo is required to explain the persistence of the geomagnetic field.

See also: → Ohmic; → decay; → time.

1. A loss of electric energy due to conversion into heat when a current flows through a resistance. Same as Ohmic loss.
   

2. In plasma physics, the energy released by charged particles as they make collisions with other particles.

See also: → Ohmic; → dissipation.

Same as → Ohmic dissipation.

See also: → Ohmic; → loss.

The puzzle of why the night sky is not as uniformly bright as the surface of the Sun if, as used to be assumed, the Universe is infinitely large and filled uniformly with stars. It can be traced as far back as Johannes Kepler (1571-1630), was discussed by Edmond Halley (1656-1742) and Philippe Loys de Chéseaux (1718-1751), but was not popularized as a paradox until Heinrich Olbers took up the issue in the nineteenth century. This paradox has been resolved by the → Big Bang theory. In a Universe with a beginning, we can receive light only from that part of the Universe close enough so that light has had time to travel from there to here since the Big Bang. The night sky is dark because the galaxies are only about ten billion years old and have emitted only a limited amount of light, not because that light has been weakened by the expansion of the Universe (P. S. Wesson et al., 1987, ApJ 317, 601).

See also: Formulated in 1826 by Heinrich Wilhelm Olbers (1758-1840),
German physician and amateur astronomer, who discovered the asteroids Pallas and Vesta as well as five comets; → paradox.

Of an astronomical object, having existed as specified with relation to younger or newer objects of the same category; e.g. → old star.

Etymology (EN): From M.E., from O.E. eald, ald; cf. Du. old, Ger. alt, Goth. altheis; akin to O.N. ala “to nourish.”

Etymology (PE): Kohan “old, ancient,” kohné “worn;” Mid.Pers. kahwan “old, aged, worn.”
Pir, from Mid.Pers. pir “old, aged, ancient;” Av. parô (adv.) “before, before (of time), in front (of space);” cf. Skt. puáh, combining form of puras “before (of time and place), in front, in advance.”

A member of a population of stars that, according to stellar evolution theories,
are almost as aged as the galaxy in which it resides.

See also: → old; → star.

A population of stars in a stellar system that have definitely left the → main sequence.

See also: → old; → stellar; → population.

A silicate mineral of magnesium (Mg₂SiO₄) and iron (Fe₂SiO₄,) found commonly in basalt and in carbonaceous chondrites.

See also: From Ger. Olivin, from olive, because of its olive-green to gray-green color,

• -in equivalent to -ine a noun suffix used in chemical and mineralogical nomenclature.

The highest peak on Mars, and the largest volcano in the solar system. It rises to a height of 27 kilometres above the datum level selected on the basis of atmospheric pressure.This gigantic shield volcano, 700 kilometres across, is similar in nature to volcanoes on Earth but its volume is at least fifty times greater than its nearest terrestrial equivalent.

See also: From L. Mons, → mountain, + Olympus, from Gk. Olympos a mountain (2966 m) in north-east Greece, on the boundary between Thessaly and Macedonia, mythical abode of the greater Grecian gods.

The largest and most luminous → globular cluster associated with the Milky Way Galaxy. Omega Centauri is located about 18,300 → light-years away and contains several million old stars. The stars in its center are so crowded that they are believed
to be only 0.1 light-year away from each other. It is about 12 billion years old. Omega Centauri was first listed in Ptolemy’s catalog nearly two thousand years ago. In 1677 Edmond Halley reported it as a nebula, and in the 1830s John Herschel was the first to correctly identify it as a globular cluster. Also called NGC 5139.

See also: Omega, Gk. alphabet letter; Centauri, → Centaurus.

In the → solar dynamo model, the process whereby
the → meridional magnetic field is stretched into an → azimuthal magnetic field by → differential rotation. See also → alpha effect.

See also: Omega (ω), Gk. letter of alphabet; → effect.

An → H II region located in the rich star fields of the → Sagittarius area of the Milky Way. Its distance from the Earth is between 5,000 and 6,000 → light-years and spans some 15 light-years in diameter. Its other designations are: Swan Nebula, Messier 17, and NGC 6618.

See also: Omega, Gk. alphabet letter; → nebula.

Another name for → Mira.

See also: Omicron, Gk. alphabet letter; Ceti, → Cetus.

A bright star of → apparent visual magnitude  V = 4.3 lying in the constellation → Lupus. Among its other designations: HD 130807, HR 5528, HIP 72683. It is an → early-type star of → spectral type B5 IV and a member of the → Scorpius-Centaurus association. Ο Lupi is in fact a → binary system whose components have an angular separation of 0.043 arcsec, corresponding to a physical separation of 5.3 → astronomical units, with a mass ratio of 0.91.

See also: → Bayer designation; Lupi, genitive of → Lupus.

All, universally.

Etymology (EN): L. omni-, combining form of omnis “all, every,” of unknown origin.

Etymology (PE): Visp-, from Mid.Pers. visp- “all;” O.Pers. visa-, vispa- “all;” Av. vīspa- “all, every, entire, universal” (vīspô.ayāra- “lasting all the days,” vīspô.vīδvah- “knowing everything, omniscient”); cf. Skt. vīśva- “all, every; whole, universal.”

Having very great or unlimited power or authority.

See also: → omni-; → power.

Same as → ubiquitous.

See also: → omni-; → present.

Having infinite knowledge, awareness, understanding, or insight.

Etymology (EN): → omni-; L. scient “knowing,”
→ science.

Etymology (PE): Visp-, → omni-, dânâ “knowing, learned,” → science; âgâh “aware,” → look.

Adverb, used as prefix denoting “in, into, onto; toward; with continuous activity,” etc.

Etymology (EN): O.E. on, variant of an “in, on, into,” from P.Gmc. (cf. Du. aan, Ger. an, Goth. ana “on, upon”), from PIE base *ano “on” (cf. Av. ana “on,” Gk. ana “on, upon,” L. an-, O.C.S. na, Lith. nuo “down from”).

Etymology (PE): Bar “on; up; upon; in; into; at; forth; with; near; before; according to” (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”).
Dar- “in,” from Mid.Pers. andar, → intra-.

The state when two or more devices are directly connected and are communicating efficiently.
Of computers, operating under the direct control of, or connected to, a main computer.

See also: → on; → line.

Preliminary reduction of observational data at a telescope simultaneously with their acquisition.

See also: → on-line; → reduction.

In comparison with → off-source observation,
an observation which is concerned with the source itself.

See also: → on-; → dource;
→ observation.

An approximation in the treatment of photoionized → H II regions, whereby secondary ionizing photons are absorbed immediately very close to their site of emission. The secondary photons, produced by → radiative recombinations directly to the → ground states, are thus ignored with respect to the ionizing photons emitted by the → exciting star. The OTS approximation requires that the ionized gas be sufficiently dense so that secondary ionizing photons are very likely absorbed within the H II region.

Etymology (EN): → on; → spot; → approximation.

Etymology (PE): Nazdineš, → approximation; dar “in,” from Mid.Pers. andar, → intra-.

1. The smallest whole number, symbol 1; unity. The → multiplicative identity for real and complex numbers.
   

2. Being or amounting to a single unit or individual or entire thing, item, or object rather than two or more; a single.

Etymology (EN): O.E. an, from P.Gmc. *ainaz (cf. O.N. einn, Dan. een, O.Fris. an, Du. een, Ger. ein, Goth. ains), PIE base *oinos, cognate with Pers. yek, as below.

Etymology (PE): Yek “one, alone,” from Mid.Pers. êwak, êv, yak, êk; cf. Baluci hivak “one, single” (Proto-Iranian *aiua-ka-); O.Pers. aiva- “one, alone;” Av. aēuua- “one, alone;” Skt. éka- “one, alone, single;” Gk. oios “alone, lonely;” L. unus “one;” E. one.

A hypothetical flow in which all the flow parameters may be expressed as functions of time and one space coordinate only. This single space coordinate is usually the distance measured along the center-line of some conduit in which the fluid is flowing (B. Massey, Mechanics of Fluids, Taylor & Francis, 2006).

See also: → one; → dimensional; → flow.

A → magnetohydrodynamics treatment in which the → plasma consists only of one particle species and moves with the bulk speed. The thermal motion of the particles is neglected and thus there is no motion of particles relative to each other.

See also: → one; → fluid; → magnetohydrodynamics.

A relationship between two distinct sets of elements such that every member of the first set can be paired with a unique element in the second set; and every member of the second set can be paired with a unique element in the first set. Two sets so related are said to be isomorphic.

See also: → one; → correspondence.

A plant, Allium cepa, of the amaryllis family, having an edible, succulent, pungent bulb (Dictionary.com).

Etymology (EN): M.E., from Anglo-French union, O.Fr. oignon “onion,” from L. unio-, also “pearl,” literally “one, unity;” sense connection is the successive layers of an onion.

Etymology (PE): Mid.Pers. payâz, payâc; cf. Sogd. piyâk “onion,” Yidgha piq, Vakhi piûk, Yazghulami piyeq, Kurd. pivaz; maybe related to PIE *peuk- “to prick, to sting, to stab,” because of its pungent taste;
cf. Gk. peukos “sting, prick, cusp, peak.”

An adverb meaning without others or anything further; alone; solely; exclusively. → if and only if.

Etymology (EN): M.E., from O.E. anlic, from ân, → one.

Etymology (PE): Ivâz, from Mid.Pers. êvâz/êvâc, from Proto-Iranian *aiua-ka-; cf. Pers. yek, → one.
Tanhâ “only,” from Mid.Pers. tanihâ “alone,” literally “by one person,” from tan “body, person;” Av. tanu- “body, person;” cf. Skt. tanu- “body, person;” perhaps related to Proto-Iranian *tan-, PIE root *ten- “to extend, stretch,” → tension.

1. Classical philosophy: That part of metaphysics which is concerned with the nature of existence or the essence of things.
   

2. Contemporary philosophy: The conceptions of existence in general, such as those dealt with in various versions of existentialism.
   

3. Informatics: A description of knowledge as a set of concepts within a domain and the relationships that can exist between those concepts. An ontology is designed for the purpose of enabling knowledge sharing and reuse.

Etymology (EN): From Mod.L. ontologia, from Gk. on (genitive ontos) “being” (pr.p. of einai “to be”), akin to L. esse “to be;” Pers. hastan, astan “to be,” hasti “existence, being,” as below; + -logia, → -logy.

Etymology (PE): Hasti-šenâsi, from hasti “existence,” from hastan “to be,” → entity, + -šenâsi, → -logy.

A huge theoretical cloud of → comets surrounding the Sun between about 50,000 and 100,000 → astronomical units
and thought to be the reservoir of → long-period comets
entering the inner solar system.

See also: Named after Jan Hendrik Oort (1900-1992), a Dutch astronomer who proposed its existence in 1950. He also made major contributions to our knowledge of the structure and rotation of the Milky Way Galaxy; → cloud.

1. The upper limit for the density of all matter in the plane of the Galaxy near the Sun’s
   locality, as calculated from the velocities and distribution of stars in relation to the gravitational field of the Galactic disk. The value is 0.14 solar masses per cubic parsec, or 9.5 x 10^-24 g cm³.
   

2. The outer boundary of the → Oort cloud. The current estimate is about 100,000 → astronomical units from the Sun, which is approximately one third of the distance to the nearest star → Alpha Centauri.

See also: → Oort cloud; → limit.

A 40-year period of unusually low → solar activity, from about 1010 to 1050. See also the → Maunder minimum.

See also: → Oort cloud; → minimum.

Two parameters, denoted A and B, that describe the major features of our Galaxy’s differential rotation in the Sun’s neighbourhood. A is one-half of the shear and equal to +14.4 ± 1.2 km s^-1 kpc^-1, and B, one-half of the vorticity, equal to -12.0 ± 2.8 km s^-1 pc^-1.

See also: → Oort cloud; → constant.

1. General: The state or quality of being opaque.
   

2. A measure of the absorption of photons on their way from the stellar center to the surface. Opacity depends upon the frequency of the radiation, the density, the chemical composition, and the thermodynamic state of the gas. For a given density, the hotter the gas the lower the opacity, since the gas absorbs less readily, as described by → Kramers’ law.
   Conversely, the cooler the gas the higher the opacity. See also → stellar pulsation, → kappa mechanism, → valve mechanism, → partial ionization zone.

Etymology (EN): From Fr. opacité, from L. opacitatem (nom. opacitas) “shade, shadiness,” from opacus “shaded, dark, opaque.”

Etymology (PE): Kederi, from keder “opaque,” from Ar. kader + -i suffix forming nouns from adjectives.

A → mineral, an amorphous form of → silica, SiO₂ with some → water of → hydration,
found in many varieties and colors, including a form that is milky white.

Etymology (EN): M.E., from M.Fr. opalle, from L. opalus, from Gk. opallios “opal, gem;” probably from a source akin to Skt. upala “jewel, precious stone.”

Etymology (PE): Loan from E., as above.

1. The state of being → opalescent, or emitting an → iridescence like that of an opal.
   

2. Physics: The milky → iridescent appearance of a dense transparent medium when it is illuminated by → polychromatic visible radiation (such as sunlight) due to local → fluctuations in its density and therefore in its → refractive index. → critical opalescence.

See also: → opal; → -escence.

Exhibiting a play of colors like that of the opal.

See also: → opal; → -escent.

General: Not permitting a radiation to pass through.
Meteo.: A condition where a material, such as a cloud, blocks the passage of radiant energy, especially light.

See also: Adj. form of → opacity.

Not closed.
Math.: → open interval; → open set.

Etymology (EN): O.E. open “not closed down, raised up,” also “uncovered, bare; plain, evident,” related to up; from P.Gmc. *upana (cf. O.N. opinn, Swed. öppen, Dan. aaben, O.Fris. epen, O.H.G. offan “open”), from PIE *upo “up from under, over;” cf. L. sub; Gk. hypo; O.Pers. upā (prep.) “under, with;” Av. upā, upa (prep.; prevb) “toward, with, on, in;”
Mod.Pers. bâ “with,” from abâ;
Skt. úpa (adv., prevb., prep.) “toward, with, under, on.”

Etymology (PE): Bâz “open,” from Mid.Pers. abâz-, apâc-, O.Pers. apa- [pref.] “away, from;” Av. apa- [pref.] “away, from,”
apaš [adv.] “toward the back;” cf. Skt. ápāñc “situated behind.”

A loose grouping of dozens or hundreds of young stars distributed in a region a few light-years across. Open clusters are relatively young, typically containing many hot, highly luminous stars. They are located within the disk of the Galaxy, whence their older name Galactic clusters.

See also: → open; → cluster.

An interval that does not include its two endpoints.

See also: → open; → interval.

In the context of solar physics, a → magnetic field line when it crosses the solar surface only once, i.e., when it goes from surface to infinity. This is the case at a sufficiently large scale in → coronal holes. This is mostly not the case in → active regions.

See also: → open; → magnetic; → field; → line.

A → set consisting of points having → neighborhoods wholly contained in the set, as the set of points within a circle.

See also: → open; → set.

A space of infinite volume without any boundary. Triangles which lie on the surface of an open space will have a sum of angles which is less than 180°. An open space has a negative → curvature. See also → open Universe, → closed space.

See also: → open; → space.

Thermodynamics: A system which can exchange both energy and matter with the surroundings. → closed system; → isolated system.

See also: → open; → system.

A → Freidmann-Lemaitre  → cosmological model in which → space is → infinite and of → negative 
→ curvature or → Euclidean, and which expands forever.

See also: → open; → universe.

In → predicate logic, a → well-formed formula with one or more → free occurrences of → variables.

See also: → open; → wff.

The capability of being put into use, operation, or practice.

See also: → operable; → -ity.

Capable of operating or of being operated.

See also: → operate; → -able.

To function or work; to make something function or work.

Etymology (EN): From L. operari “to work, labor,” L. opus “a work, labor, exertion,” Av. *āpah-, *apah- “to do, operate,” see below, Skt.
ápas- “work, action, religious act;”
O.H.G. uoben “to start work, to practice, to honor;” Ger. üben “to exercise, practice;” Du. oefenen; O.E. æfnan “to perform, work, do,” afol “power”); PIE base *op- “to work, perform.”

Etymology (PE): Âpâridan, from âpâr-, from Av. *āp(ah)- “to do, operate,” as above, + suffix -ar (as in vadar- “weapon,” zafar- “jaw,” baēvar- “thousand,” and so on), shifted to -âr, + -idan suffix of infinitives. The Av. *āpah- “to do, operate,” is extant in Mod.Pers. xub “good;” Mid.Pers. hwp, xub “good;” from Av. huuāpah- “doing good work, masterly,” from huu-, hv- “good” → eu- + āpah- “work, deed,” hauuapanha- “creativity;” cf. Skt. sv-ápas- “doing good work, skillful;” PIE base *op-, as above.

The program that, after being initially loaded into the → computer by a boot program, → manages all the other → programs in a computer.

See also: → operating; → system.

1. General: An act or instance, process, or manner of functioning or operating.
   

2a) Math.: A mathematical process, as addition, multiplication, or differentiation.

2b) The action of applying a mathematical process to a quantity or quantities.

3. Computers: An action resulting from a single instruction.

See also: Verbal noun of → operate

Pertaining to a process or series of actions for achieving a result.

See also: Adj. of → operation.

A method of mathematical analysis which in many cases makes it possible to reduce the study of differential operators, pseudo-differential operators and certain types of integral operators,
and the solution of equations containing them, to an examination of simpler algebraic problems. It is also known as operational analysis.

See also: → operational; → calculus.

In the philosophy of science, the view that → concepts are defined in terms of measuring operations which determine their applicability. Same as operationism.

See also: → operation; → -ism.

Math.: Something that acts on another function to produce another function. In linear algebra an “operator” is a linear operator. In calculus an “operator” may be a differential operator, to perform ordinary differentiation, or an integral operator, to perform ordinary integration.

See also: From → operate; + → -or.

A small satellite of → Uranus, the second nearest to the planet, discovered from the images taken by the Voyager 2 spacecraft in 1986. Also denoted Uranus II, it has a diameter of 32 km. Ophelia is one of the two → shepherd moons that keep the planet’s Epsilon ring, the other being → Cordelia.

See also: Ophelia is the daughter of Polonius in Shakespeare’s Hamlet.

The Serpent Holder. An extensive constellation located in the equatorial regions of the sky at about 17h 20m right ascension, 5° south declination. Although this constellation is not part of the zodiac, the Sun passes through it in December each year. Ophiuchus contains five stars of second magnitude and seven of third magnitude. Other designations: Serpent Bearer, Serpentarius.
Abbreviation: Oph, genitive: Ophiuchi.

Etymology (EN): L. Ophiuchus, from Gk. ophioukhos “holding a serpent,” from ophis “serpent” + echein “to hold, have, keep.” The most recent interpretation is that the figure represents the great healer
Asclepius, a son of the god Apollo, who learned the secrets of keeping death at bay after observing one serpent bringing another healing herbs. To prevent the entire human race from becoming immortal under Asclepius’ care, Zeus killed him with a bolt of lightning, but later placed his image in the heavens to honor his good works.

Etymology (PE): Mâr-afsâ “a tamer or charmer of serpents; one who cures the snake-bitten by incantation,” from mâr “snake, serpent” (Mid.Pers. mâr “snake;” Av. mairya- “snake, serpent”) + afsâ agent noun of afsâyidan, from afsun “incantation” (Mid.Pers. afsôn “spell, incantation,” afsûdan, afsây- “to enchant, protect by spell”).

To hold or express an opinion.

See also: Verb for → opinion.

1. A belief or judgment that rests on grounds insufficient to produce complete certainty.
   

2. A personal view, attitude, or appraisal.
   

3. Law: The formal statement by a judge or court of the reasoning and the principles of law used in reaching a decision of a case (Dictionary.com).

Etymology (EN): M.E., from O.Fr., from L. opinion- “opinion, conjecture; appreciation,” from opinari “to think, judge, suppose,” from PIE *op- “to choose.”

Etymology (PE): Pažân, from Pashto pažân / pêžân, from prefix- pa-, originally *pati-, + žân variant of Pers. dân-, dânestan “to know,” zân as in farzâne “intelligent, wise;” Balochi zân, Kurd. zân “to know;” cf. Sogd. patzân, Khotanese paysân- “to know;” Yidgha and Munji vəzân “to know;” Yizghulami vəzan, Yaghnobi bīzon, Sarigholi pajan, Ormuri pazán “to know;” Av. paiti-zan- “to recognize, acknowledge, appreciate;” from prefix paiti- + zan- “to know, have knowledge;”
Mod.Pers. dân-, dânestan “to know,” variant šenâxtan “to know, recognize,” → science.

The upper bound to the mass of a → neutron star, the mass beyond which the pressure of neutron → degenerate matter is not capable of preventing the → gravitational collapse which will lead to the formation of a → black hole. Modern estimates range from approximately 1.5 to 3.0 → solar masses. The uncertainty in the value reflects the fact that the → equation of state for → overdense matter is not well-known.

See also: Oppenheimer, J.R., Volkoff, G.M., 1939, Physical Review 55, 374. Named after Robert Oppenheimer (1904-1967), an American theoretical physicist, and
George Volkoff (1914-2000), a Canadian physicist, who first calculated this limit.
Oppenheimer is widely known for his role as the scientific director of the Manhattan Project,
the World War II effort to develop the first nuclear weapons at the secret
Los Alamos laboratory in New Mexico; → limit.

A person who is on an opposing side in a game, contest, controversy, or the like; adversary (Dictionary.com).

Etymology (EN): L. opponent-, p.p. of opponere “to oppose, to object to,” literally “set against, set opposite,” from op- variant of ob- before p
“against” + ponere “to put, set, place,” → position.

Etymology (PE): Pâdistgar, from pâdist, → opposition, + -gar, → -or.

1. Appropriate, favorable, or suitable.
   

2. Occurring or coming at an appropriate time; well-timed (Dictionary.com).

Etymology (EN): M.E., from O.Fr. opportun and directly from L. opportunus “fit, convenient, suitable,” from the phrase ob portum veniens
“coming toward a port,” literally “a wind blowing to harbor,” from ob “to, toward” + portus “access, harbor.”

Etymology (PE): Nikvâ “appropriate, suitable,” from nik, nêk, neku “good, beautiful, elegant;” Mid.Pers. nêk, nêvak, nêkôg “good, beautiful;” O.Pers. naiba-

• -vâ relation suffix (as in pišvâ, pilévâ); alternatively, nikvâ “good/appropriate wind,” from nik + vâ “wind,” variant of bâd, → wind, in several dialects.

The policy or practice, as in politics, business, or one’s personal affairs, of adapting actions, decisions, etc., to expediency or effectiveness regardless of the sacrifice of ethical principles (Dictionary.com).

See also: → opportune + → -ism.

A person who adapts his actions, responses, etc, to take advantage of opportunities, circumstances, etc. (Dictionary.com).

See also: → opportune + → -ist.

1. An appropriate or favorable time or → occasion.
   

2. A situation or condition favorable for attainment of a goal (Dictionary.com).

See also: → opportune + → -ity.

1. To act against or provide resistance to.
   

2. To stand in the way of; hinder; obstruct (Dictionary.com).

Etymology (EN): M.E., from O.Fr. oposer “to oppose, resist; contradict,” from poser “to place, lay down,” blended with L. opponere “to oppose, to object,” → position.

Etymology (PE): Pâdistidan, infinitive from pâdist, → opposition.

1. Situated, placed, or lying face to face with something else or each other, or in corresponding positions with relation to an intervening line, space, or thing: opposite ends of a room (Dictionary.com).
   

2. Contrary or radically different in some respect common to both, as in nature, qualities, direction, result, or significance; opposed (Dictionary.com).
   

3. An → antonym.

Etymology (EN): M.E., from M.Fr., from L. oppositus, p.p. of opponere,
→ opposition.

Etymology (PE): 1) Ru-be-ru “face to face,” → surface.

2. Pârun, short for pâdrun, from pâd-, → counter, + -run “side, direction” (as in birun, darun, vârun), → out.
   

3. Pâdcem, → antonym.

1. The action of opposing, resisting, or combating.
   

2. A person or group of people opposing, criticizing, or protesting something, someone, or another group (Dictionary.com).
   

3. The position of a solar system body having its orbit outside that of the Earth when the Earth is in a line between the Sun and the body. At opposition the body has a solar → elongation of 180°, and is closest to the Earth. It
   will, in principle, be visible throughout the night. It will rise in the east as the Sun sets in the west and it will set as the Sun rises. This is because, at opposition, the body and the Sun are 12 hours apart. The inner planets can never be in opposition. The opposite of opposition is → conjunction.
   

4. Two periodic quantities of the same frequency are said to be in opposition when the → phase difference between them is one half of a → period.

Etymology (EN): Verbal noun of → oppose.

Etymology (PE): Pâdist “standing against,” from pâd- “agaist, contrary to,” → anti-,

• ist present stem of istâdan “to stand” (Mid.Pers. êstâtan, O.Pers./Av. sta- “to stand; to set;” Av. hištaiti; cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh,” stasis “a standing still;” L. stare “to stand;” Lith. statau “place;” Goth. standan; PIE base *sta- “to set, stand”).
  Pâdistân, from pâdist + -ân suffix of place and time.

To burden with cruel or unjust impositions or restraints; subject to a burdensome or harsh exercise of authority or power (Dictionary.com).

Etymology (EN): M.E. oppressen, from O.Fr. opresser “oppress, torment, smother,” from M.L. oppressare, from L. opprimere “press against, press together, press down; subdue, prosecute relentlessly,” from op variant of ob “against” + premere “to press, hold fast.”

Etymology (PE): Infinitive from setam, → oppression.

1. The exercise of authority or power in a burdensome, cruel, or unjust manner.
   

   2. An act or instance of oppressing or subjecting to cruel or unjust impositions or restraints.
      

   3. The state of being oppressed (Dictionary.com).

Etymology (EN): M.E. oppressioun, from O.Fr. opresser “oppress; torment,” from M.L. oppressare, from L. opprimere “press against, press down;” from op, variant of ob “against”

• premere “to press, hold fast.”

Etymology (PE): Setam, from Mid.Pers. sthmbk / stambag / “oppressive; obstinate,” related to sitabr “strong, firm,” staft “hard; firm, strong; fierce,” Pers. seft “firm, hard, tight;” sitanbah “strong, robust, bold;” Av. aša.stəmbana- “having the support/firmness of aša;” Lith stembti “to oppose;” Gk. astemphes “unshakable.”

To make a choice; choose (usually followed by for).

Etymology (EN): From Fr. opter “to choose,” from L. optare “to choose, desire, wish for,” from L. optare “to desire, choose,” from PIE root *op- “to choose, prefer.”

Etymology (PE): Optidan, from L. optare, as above.

Of, relating to, or constituting a verbal mood that is expressive of wish or desire.

Etymology (EN): From M.Fr. optatif, from L. optativus, from optatus, p.p. of optare, → opt, + -ivus, → -ive.

Etymology (PE): Optâné, optmand, from opt present stem of optidan, → opt, + adj. suffixes -âné, -mand. -yi.

1. Of or pertaining to the eye or sight.
   
2. Same as → optical.

Etymology (EN): From M.Fr. optique, from M.L. opticus “of sight or seeing,” from Gk. optikos “of or having to do with sight,” from optos “seen, visible,” from op-, root of opsesthai “be going to see,” related to ops “eye,” from PIE *ok^w- “eye/see.”

Etymology (PE): 1) Nuri, nurik, from nur, → light + -i, -ik adj. suffix → -ic.
2) Didgâni, related to didgân “eyes,” plural of didé “eye; seen,” from didan “to see” (Mid.Pers. ditan “to see, regard, catch sight of, contemplate, experience;” O.Pers. dī- “to see;” Av. dā(y)- “to see,” didāti “sees;” cf.
Skt. dhī- “to perceive, think, ponder; thought, reflection, meditation,” dādhye; Gk. dedorka “have seen”).

The direction in a doubly refracting crystal in which light is propagated without double refraction.

See also: → optic; → axis.

1. Of, pertaining to, or applying optics or the principles of optics.
   
2. Of or pertaining to sight or vision; visual; of or pertaining to the eye.

Etymology (EN): From → optic + → -al.

Etymology (PE): → optic.

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

See also: → optical; → aberration.

The property possessed by some substances and their solutions of rotating the plane of vibration of → polarized light. When a beam of → linearly polarized light is sent through an optically active substance, such as crystalline quartz and sugar solution, the direction of vibration of the emerging linearly polarized light is found to be different from the original direction. Those which rotate the → plane of polarization to the right, for an observer looking in the incoming beam, are called → dextrorotatory or right handed; those which rotate it to the left, → levorotatory or left handed. Optical activity may be due to an asymmetry of molecules of a substance (solutions of cane sugar) or it may be a property of a crystal as a whole (crystalline quartz).

See also: → optical; → activity.

The mathematical evaluation of an optical system to determine its basic optical properties and image quality characteristics.

See also: → optical; → analysis.

An instrument used to test lenses by utilizing the → optical transfer function. It consists of a HeNe laser, a beamsplitter and two mirrors.

See also: → optical; → autocorrelator.

Line passing through the optical center and the center of curvature of a → spherical mirror or → lens. Same as → principal axis.

See also: → optical; → axis.

A track or table on which sources, lenses, mirrors, and other optical components can be mounted and moved. It is used in optics experiments.

Etymology (EN): → optical; bench, M.E., from O.E. benc “long seat;” cf. Da. bænk, M.Du. banc, O.H.G. banch.

Etymology (PE): Miz “table,” originally “preparations for entertaining a guest; guest;” Mid.Pers. mêzd “offering, meal;” nurik, nuršenâxti, → optical.

Of a thin lens, a point situated at the geometrical center of the lens, through which an incident ray passes without being deviated.

See also: → optical; → center.

Any device such as a → lens, → prism, → mirror, and/or other similar objects used in an → optical system.

See also: → optical; → component.

An astronomical object with usually weak emission in the → visible found to be the optical representation of the object radiating chiefly in other
→ wavelengths of the → electromagnetic spectrum.

See also: → optical; → counterpart.

The transmittance of a point on a photographic negative equal to the log to the base 10 of the reciprocal of the transmittance through the negative at that point.

See also: → optical; → density.

1. A measure of how much radiation is absorbed when traveling through a medium (such as the atmosphere of a star or the interstellar medium) from the source to a given point. It depends on the type of medium and the frequency of radiation. It is defined as a dimensionless quantity dτ_λ = κ_λ ρ dx, where κ_λ is the extinction coefficient, ρ the density, and dx the path length.
   

2. The ratio of the intensity of radiation (light or radio waves) incident on a ring to that emerging from the opposite face of the ring, expressed as a natural logarithm. If the reduction in intensity is by a factors of e (= 2.718), the ring is said to have an optical depth of 1. Normal (or normalized) optical depth is optical depth corrected for oblique (non-vertical) viewing. In ring studies, the terms optical depth and optical thickness are generally used interchangeably. These terms generally refer to a particular wavelength (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → optical; → depth.

A pair of stars that lie close to each other in the sky by chance, but are not physically associated, in contrast to a true → binary star.

See also: → optical; → double star.

A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote internal reflection. It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.

See also: → optical; → fiber.

A perception of visual stimuli in which what is perceived is in a way different from the way it is in reality. Same as visual illusion.

See also: → optical; → illusion.

An instrument that either processes light waves to enhance an image or analyzes light waves to determine one of a number of characteristic properties.

See also: → optical; → instrument.

An → astrophysical jet which is visible in the optical range of the electromagnetic radiation.

See also: → optical; → jet.

(polar flattening) The ratio of the difference between equatorial and polar diameters to the equatorial diameter. A sphere has an oblateness of 0; an infinitely thin disk has an oblateness of 1 (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → optical; → oblateness.

In → geometric optics, the distance a light ray would travel in a vacuum in the same time it travels from one point to another, a specified distance, through one or more optical media.

See also: → optical; → path.

A member of a rare class of pulsars, such as the → Crab pulsar and → Vela pulsar, which can be detected in the visible spectrum.

See also: → optical; → pulsar.

A process in which light energy is used to raise electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction, to pump the active laser medium so as to achieve population inversion. The technique was developed by 1966 Nobel Prize winner Alfred Kastler in the early 1950’s.

See also: → optical; → pumping.

A collection of lens, prisms, mirrors, and/or other devices, placed in some specified configuration, to act on light (reflect, refract, disperse, polarize, etc.) and perform some definite optical function.

See also: → optical; → system.

Same as → optical depth.

See also: → optical; → thickness.

The function that provides a full description of the imaging quality of an optical system. A combination of the → modulation transfer function (MTF) and the → phase transfer function (PTF) , the OTF describes the spatial (angular) variation as a function of spatial (angular) frequency.

See also: → optical; → transfer; → function.

The optical portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground.

See also: → optical; → window.

Adverb of → optical.

Etymology (EN): → optical + -ly.

Etymology (PE): Nurâné, from nur, → light, + -âné “-ly;” nurikâné, from nurik, → optical, + -âné “-ly.”

Relating to → optical activity.

See also: → optically; → active.

The qualifier of a medium in which the → optical depth is large,
significantly larger than 1.

See also: → optically; → thick.

A wind with the → sonic point located at large optical depth for continuum. In particular, → Wolf-Rayet star winds are → optically thick. However, the outer parts of W-R winds are → optically thin for continuum, and in those regions the matter flow is driven by the same mechanism as in the winds of OB stars.

See also: → optically; → thick; → wind.

The qualifier of a medium in which the → optical depth is large,
significantly smaller than 1.

See also: → optically; → thin.

A member of a small subset of quasars consisting of bright radio galaxies whose flux of visible light output can vary by as much as 50% in a single day.

See also: → optically; → violent; → variable; → quasar.

1. A maker or seller of optical glass.
   
2. A person specialized in optical instruments.

Etymology (EN): From Fr. opticien, from M.L. optic(a), → optics, + -ien “-ian.”

Etymology (PE): 1) Eynak-sâz “eyeglass maker,” from eynak→ eyeglasses + sâz agent noun of sâxtan, sâzidan “to build, make, fashion; to adapt, adjust, be fit” (from
Mid.Pers. sâxtan, sâz-, Manichean Parthian s’c’dn “to prepare, to form;” Av. sak- “to understand, to mark,” sâcaya- (causative) “to teach”).

2. Nurikâr, nurik-kâr “optics performer, optics workman, optics specialist,” from nurik, → optics, + -kâr
   variants -gar,-gâr,
   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 branch of physics that deals with the properties and phenomena of electromagnetic radiation in the wavelength range extending from the ultraviolet (at about 40 nm) to the far-infrared (at 1 mm) and with vision.

Etymology (EN): Optics, from optic, from M.Fr. optique, from M.L. opticus “of sight or seeing,” from Gk. optikos “of or having to do with sight,” from optos “seen, visible,” from op-, root of opsesthai “be going to see,” related to ops “eye,” from PIE *ok^w- “eye; to see” (→ eye); → -ics.

Etymology (PE): Nurik, from nur, → light, + -ik→ -ics. Nuršenâsi, from nur, → light, + šenâsi→ -logy.

One that minimizes or maximizes some quantity or combination of quantities,
such as time, energy, distance, path, etc.

See also: Optimal, adj. of → optimum.

General: The fact of optimizing. The condition of being optimized.
→ optimize.
Computer science: Improving a system to reduce runtime, bandwidth, memory requirements, or other property of a system.

See also: Verbal noun of → optimize

General: To make as effective, perfect, or useful as possible.
Math.:

To determine the maximum or minimum values of a specified function by systematically choosing the values of the variables from within an allowed set.

Etymology (EN): From opti(mum), → optimum, + → -ize.

Etymology (PE): Behinidan, from behin, → optimum, + -idan infinitive suffix. Behin sâxtan compound verb from behin + sâxtan, sâzidan “to build, make, fashion; to adapt, adjust, be fit” (from
Mid.Pers. sâxtan, sâz-, Manichean Parthian s’c’dn “to prepare, to form;” Av. sak- “to understand, to mark,” sâcaya- (causative) “to teach”).

In computer science, a program that optimizes the speed or resource utilization. → optimize.

See also: Agent noun of → optimize.

The best or most favorable point, degree, amount, etc. The greatest degree or best result obtained or obtainable under specific conditions.

Etymology (EN): From L. optimum, neuter singular of optimus “best” (used as a suppletive superlative of bonus “good”).

Etymology (PE): Behiné, from behin superlative of beh “good, fine” (Mid.Pers. veh “better, good;” O.Pers. vahav-, vahu-; Av. vah-, vohu- “good;” cf. Skt. vasu- “good;” Hittite wasu-; Gaulish vesus “good”) + -in superlative suffix + -é nuance suffix.

1. The power or right of choosing.
   

2. Something that may be or is chosen; choice.
   

3. The act of choosing (Dictionary.com).

See also: → opt; → -tion.

Left to one’s choice; not required or mandatory (Dictionary.com).

See also: → option; → -al.

Any of various instruments for measuring refractive errors of the eye.

Etymology (EN): From opto- “vision; eye; optis,” combining form from Gk. optos “seen, visible,” from opsesthai “be going to see,” related to ops “eye;” → -meter.

Etymology (PE): Did, → vision; -sanj, → -meter.

A → conjunction word used to indicate alternatives.

Etymology (EN): M.E., from or, adverb “early, before,” from Old Norse ār akin to O.E. ær “early.”

Etymology (PE): Yâ, from Mid.Pers. ayâb, aviâp “or;” cf. P.Pers. ada, Av. adā, aδa “then.”

1. A globose berry with a yellowish to reddish-orange rind and a sweet edible pulp.
   

2. Any of a group of colors that are between red and yellow in hue (Merriam-Webster.com).

Etymology (EN): M.E., from O.Fr. orange, orenge, from M.L. pomum de orenge, from It. arancia, originally narancia, alteration of Ar. nâranj, from Pers. nârang, from Skt. narangah “orange tree.”

Etymology (PE): Nârenji, from nârenj “orange,” as above.

The path followed by a body moving in a gravitational field. For bodies moving under the influence of a centrally directed force, without significant perturbation, the shape of the orbit must be one of the conic section family of curves (circle, ellipse, parabola, or hyperbola).

Etymology (EN): L. orbita “wheel track, course, circuit.”

Etymology (PE): Madâr, from Ar.

A gradual change in the orbit of a spacecraft caused by aerodynamic drag of a planet’s outer atmosphere and other forces.

See also: → orbit; → decay.

1. Of or relating to an orbit.
   
2. A wave function that describes the state of an electron with a given energy (n, l, and m_l quantum numbers) in an atom (atomic orbital) or in a molecule (molecular orbital).

See also: Orbital, from → orbit + → -al.

1. Mechanics: The → angular momentum associated with the motion of a particle about an origin, equal to the cross product of the position vector (r) with the linear momentum (p = mv): L = r x p. Although r and p are constantly changing direction, L is a constant in the absence of any external force on the system. Also known as orbital momentum.
   

2. Quantum mechanics: The → angular momentum operator associated with the motion of a particle about an origin, equal to the cross product of the position vector with the linear momentum, as opposed to the → spin angular momentum. In quantum mechanics the orbital angular momentum is quantized. Its magnitude is confined to discrete values given by the expression: ħ &radic l(l + 1), where l is the orbital angular momentum quantum number, or azimuthal quantum number, and is limited to positive integral values (l = 0, 1, 2, …). Moreover, the orientation of the direction of rotation is quantized, as determined by the → magnetic quantum number. Since the electron carries an electric charge, the circulation of electron constitutes a current loop which generates a magnetic moment associated to the orbital angular momentum.

See also: → orbital; → angular; → momentum.

The → perpendicular to the → orbital plane.

See also: → orbital; → axis.

Objects in orbit around Earth created by humans, that no longer serve any useful purpose. They consist of everything from entire spent rocket stages and defunct satellites to explosion fragments, paint flakes, dust, and slag from solid rocket motors, and other small particles. Also called space junk and space waste.

See also: → orbital; → debris .

An electron contained within an atom which may be thought of as orbiting around the nucleus, in a manner analogous to the orbit of a planet around the Sun.

See also: → orbital; → electron .

Any of the six parameters needed to specify the → orbit of an object around a → primary body (such as a planet around the Sun or a satellite around the Earth) and give its position at any instant.

Two of them define the size and the form of the orbit: → semi-major axis (a) and → eccentricity (e).

Three angular values determine the orbit position in space: the → inclination (i) of the object’s → orbital plane to the reference plane (such as the → ecliptic), the → longitude of ascending node (Ω), and the → argument of periapsis (ω). And finally the sixth element is the → time of periapsis passage which allows calculating the body’s position along the orbit at any instant.

See also: → orbital; → element.

The → sum of the → potential energy and the → kinetic energy of an object in → orbit.

See also: → orbital; → energy.

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. The orbital inclination of the Earth’s orbit is 0°; those of Mercury, Venus, and Mars are 7.01°, 3.39°, and 1.85° respectively.

See also: → orbital; → inclination .

The moving of a spacecraft between two different orbits resulting from a change in its velocity (acceleration). Generally, manoeuvres are caused by
→ thrust from the spacecraft’s motors.

See also: → orbital; → maneuver.

→ orbital maneuver.

See also: → orbital; → manoeuvre.

Theoretical prediction according to which a → giant planet, formed in the outer regions of a → protoplanetary disk, could migrate inward by losing → energy and → angular momentum as the result of → gravitational interactions with the remnants of the disk. This orbital migration could explain the presence of giant gaseous Jupiter-like planets (→ hot Jupiters) very close to their host stars.

See also: → orbital; → migration.

One of the two points of intersection of the orbit of a secondary body with the plane of reference through the primary.

See also: → orbital; → node.

→ orbital element.

See also: → orbital; → parameter.

The time interval between two successive passages of an object through the same point in its orbit around another object.

See also: → orbital; → period .

In → photometry of
→ binary stars or → two-body systems, the number of whole or fractional orbits completed, from the point the photometry begins. The point is conventionally chosen as the position at which the → primary star eclipses the → secondary star, and therefore the → light curve is at a minimum. The phase keeps counting indefinitely, thus the secondary star gets eclipsed at phase 0, 1, 2, 3, and so on. At these phases the primary lies between the secondary and the observer. An orbital phase of 0.5 corresponds to halfway through the binary orbit, 0.75 is three-quarters the way through, and so on.

See also: → orbital; → phase.

The photometric variability induced by the → orbital motion in a → two-body system.

See also: → orbital; → phase.

The plane defined by the motion of an object about a primary body.

See also: → orbital; → plane .

Same as → relativistic precession.

See also: → orbital; → precession.

The situation in which two orbiting objects exert a regular, periodic gravitational influence on each other and therefore their orbital frequencies are related by a ratio of two small → integers. Orbital resonance often results in an unstable interaction in which bodies exchange momentum and shift orbits until the resonance disappears. The resonance increases the eccentricity until a body approaches a planet too closely and the body is slung away.

See also: → orbital; → resonance.

The lessening in size of the orbit of a binary system composed of two compact objects (pulsars/black holes) due to loss of energy by the system, in particular through gravitational wave radiation. This loss will cause the two objects to approach closer to each other, the orbital period decreases and the binary companions will eventually merge.

Etymology (EN): → orbital; shrinkage, from shrink, from M.E. schrinken, O.E. scrincan, from P.Gmc. *skrenkanan (cf. M.Du. schrinken, Swed. skrynka “to shrink.”

Etymology (PE): Darhamkešidegi “shrinking, shriveling,” from state noun of < i>darhamkešidé, from darham- “together, in eachother, toward eachother” (For etymology of dar-, → in-; for etymology of ham-, → com-)

• kešidé “drawn, shrivelled, wrinkled,” from Mod./Mid.Pers. kešidan, kašidan “to draw, protract,
  trail, drag, carry,” dialectal Yaqnavi xaš “to draw,” Qomi xaš “streak, stria, mark,” Lori kerr “line;”
  Av. karš- “to draw; to plow,” karša- “furrow;” Proto-Iranian *kerš-/*xrah- “to draw, plow;” cf. Skt. kars-, kársati “to pull, drag, plow;”
  Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow;” madâri, → orbital.

Same as → orbital velocity.

See also: → orbital; → speed.

The velocity of an object in a given orbit around a gravitating mass. For a perfect circular orbit, the velocity is described by the formula V =√[G(M + m)/r], where G is the gravitational constant, M the mass of the primary gravitating body, m the mass of the orbiting object, and r the radius of the orbit.

See also: → orbital; → velocity.

A → spacecraft or → satellite designed to orbit a planet or other → solar system body.

See also: → orbit; → -er.

In astronautics, the study of satellite orbits and precise
determination of orbital elements which gives the exact position of the satellite.

See also: → orbital + → -graphy.

1a) General: The way in which several items are arranged, as an indication of their relative importance or size or when each will be dealt with.

1b) Optics: → order of interference.

1c) Math.: The number of rows or columns of a
→ square matrix or → determinant.

1d) Math.: The highest → derivative appearing in a given → differential equation. For example,
d²y/dx² + 2y (dy/dx) - 3 = 0
is a differential equation of order two.

2. To arrange, regulate, or dispose things in their proper places. See also: → disorder, → first-order differential equation, → first-order partial derivative, → first-order spectrum, → interference order, → order of a tensor, → order of interference, → order of magnitude.

Etymology (EN): From O.Fr. ordre, from earlier ordene, from L. ordinem (nominative ordo) “row, rank, arrangement.”

Etymology (PE): Râyé, noun related to râyânidan “to regulate, set in order,” from Mid.Pers. râyânīdan “to arrange, organize,” from rây-
(Mod.Pers. ârây-, ârâyeš, ârâyidan “to arrange, adorn”),
ârây-, ârâstan “to arrange, adorn;” O.Pers. rād- “to prepare,” rās- “to be right, straight, true,” rāsta- “straight, true” (Mod.Pers. râst “straight, true”); Av. rāz- “to direct, put in line, set,” razan- “order;” Gk. oregein “to stretch out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” Skt. rji- “to make straight or right, arrange, decorate;” PIE base *reg- “move in a straight line;” see also → direct.

The → number of → vertices.

See also: → order; → graph.

The maximum number of the indices (→ index) of a tensor.

See also: → order; → tensor.

A whole number which characterizes a particular position of an interference fringe according to whether there is interference arising from one, two, three, etc. wavelength difference of path. Same as → interference order

See also: → order; → interference.

Value of a number or of a physical quantity given roughly, usually expressed as a power of 10. Thus, 2.5 x 10⁵ and 6.4 x 10⁵ are of the same order of magnitude, and 2 x 10⁷ is 2 orders of magnitude greater than either.

See also: → order; → magnitude.

Characterized by → order; arranged according to a rule. → ordered tree.

See also: → order + -ed.

In → graph theory, a → tree in which the → children of each → vertex are ordered.

See also: → ordered; → tree.

1. Of or relating to an order, as of animals or plants.
   

2. Of or relating to order, rank, or position in a series.
   

3. → ordinal number.

Etymology (EN): M.E., from O.Fr., from L.L. ordinalis “denoting order or place in a series,” from L. ordo (genitive ordinis) “row, series,” → order; → -al.

Etymology (PE): → order; → -al.

1. A number which defines the position or rank of something in a series, in contrast to → cardinal number. For example, first, second, tenth, etc.
   

2. In → set theory, the order type of a → well-ordered set. Also called ordinal.

See also: → ordinal; → number.

Usual; normal. → ordinary ray;
→ ordinary year.

Etymology (EN): M.E. ordinarie, from O.Fr. ordinarie, from L. ordinarius “regular, usual, orderly,” from ordo (genitive ordinis) “order”

• -arius-ary.

Etymology (PE): Šunik “ordinary,” from Mid.Pers. šônik, šônig “ordinary, customary,” from šôn “kind, manner, sort, way”

• -ik, → -ic.

A → differential equation in which the unknown function depends on only one → independent variable, as contrasted with a → partial differential equation.

See also: → ordinary; → differential; → equation.

The point M₀(x₀,y₀) of the curve F(x,y) = 0, where at least one of the partial derivatives ∂F/∂x and ∂F/∂y does not vanish. → singular point

See also: → ordinary; → point.

The ray that has an → isotropic speed in a → doubly refracting crystal. It obeys → Snell’s law upon refraction at the crystal surface.
See also → extraordinary ray

See also: → ordinary; → ray.

A → calendar year that contains 365 days and therefore is not a → leap year.

See also: → ordinary; → year.

Math.: In plane Cartesian coordinates, the distance of a point from the x-axis measured parallel to the y-axis. → abscissa.

Etymology (EN): Ordinate, from N.L. (linea) ordinate (applicata) “(line applied) in order;” from ordinatus “arranged.”

Etymology (PE): Ârâ, from ârâstan “to arrange, to set in order, adorn,”
Mid.Pers. ârây-, ârâstan “to arrange, adorn;” O.Pers. rās- “to be right, straight, true,” rāsta- “straight, true” (Mod.Pers. râst “straight, true”), rād- “to prepare;”
Av. rāz- “to direct, put in line, set,” razan- “order;” Gk. oregein “to stretch out;” L. regere “to lead straight, guide, rule,” p.p. rectus “right, straight;” Skt. rji- “to make straight or right, arrange, decorate;” PIE base *reg- “move in a straight line.”

A natural deposit containing a mineral of an element to be extracted.

Etymology (EN): Ore, merger of M.E. ore, O.E. ora “ore, unworked metal” and M.E. or(e) “ore, metal,” O.E. ar “brass, copper, bronze” (cf. O.N. eir “brass, copper;” Ger. ehern “brazen;” Erz “oar;” Goth. aiz “bronze;” O.H.G. ēr “ore”), from PIE
*aus- “gold;” cf. Mod/Mid..Pers. âhan “iron;” Av. aiianhaēna- “made of metal,” from aiiah- “metal;” Skt. áyas- “iron, metal;” L. aes “brass”

Etymology (PE): Kâné, from kân “mine,” from kandan “to dig” (Mid.Pers. kandan “to dig;” O.Pers. kan- “to dig,” akaniya- “it was dug;” Av. kan- “to dig,” uskən- “to dig out” (→ ex- for prefix us-); cf. Skt. khan- “to dig,” khanati “he digs”).

1. Biology: A structure consisting of tissues and performing some specific function in an organism, e.g. the heart, kidneys, liver, stomach.
   
2. A large musical keyboard instrument producing a wide range of musical effects using compressed air passed through pipes.
   
3. An organization or body acting on behalf of a larger institution.
   
4. A newspaper or magazine regarded as a means of communication, especially one communicating the views of a particular group such as a political party.

Etymology (EN): O.E. organe, from O.Fr. orgene “musical instrument,” from L. organa, pluriel of organum, from Gk. organon “implement, musical instrument, organ of the body,” literally “that with which one works,” from PIE *werg-ano-, from base *werg- “to do, to work” related to Gk. ergon “work” (cf. Av. varəz- “to work, do, perform, exercise;” Mod.Pers. varz-, varzidan “to labor, exercise, practise;” Arm. gorc “work;” Lith. verziu “tie, fasten, squeeze,” vargas “need, distress;” Goth. waurkjan; O.E. wyrcan “work,” wrecan “to drive, hunt, pursue”).

Etymology (PE): 1) Andâm, from Mid.Pers. handam “member, limb;” Av. handāma- “limb;” from Proto-Iranian *ham-dāman-, from prefix ham-→ com- + *dāman- “created;” Av. dā- “to give, grant; to put; to create; to determine” (O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives;” Mod.Pers. dâdan “to give;” cf.
Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer,” facere “to do, to make;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”); cf. Skt. sandháy- “joint, articulation.”

2), 3), 4) loanwords from Fr., as above.

1. Of, relating to, or derived from living organisms: organic matter.
   
2. Characterized by the systematic arrangement of parts; organized; systematic.
   
3. Chemistry: Of or designating carbon compounds.

Etymology (EN): From L. organicus, from Gk. organikos “of or pertaining to an organ,” from organon “instrument,” → organ, + → -ic.

Etymology (PE): 1) Orgânik, loan from Fr.; âli loan from Ar.; 2) sâzmândâr, from sâzmân, → organization, + dâr “having, possessor” (from dâštan “to have, to possess,” Mid.Pers. dâštan, O.Pers./Av. root dar- “to hold, keep back, maitain, keep in mind,” Skt. dhr-, dharma- “law,”
Gk. thronos “elevated seat, throne,” L. firmus “firm, stable,” Lith. daryti “to make,” PIE *dher- “to hold, support”).

The study of compounds that contain carbon chemically bound to hydrogen.

See also: → organic; → chemistry.

A molecule that is normally found in or produced by living systems. Organic molecules typically consist of carbon atoms in rings or long chains, where other atoms (e.g. hydrogen, oxygen, and nitrogen) are attached, except for → carbon monoxide (CO) and → carbon dioxide (CO₂), etc.

See also: → organic; → molecule.

1. Biology: A living individual composed of mutually interdependent parts that maintain various vital processes.
   
2. Any organized body or system conceived of as analogous to a living being. → microorganism