1590s, "conception, mental scheme," from Late Latin theoria (Jerome), from Greek theōria "contemplation, speculation; a looking at, viewing; a sight, show, spectacle, things looked at," from theōrein "to consider, speculate, look at," from theōros "spectator," from thea "a view" (see theater) + horan "to see," which is possibly from PIE root *wer- (3) "to perceive." Philosophy credits sense evolution in the Greek word to Pythagoras.
Earlier in this sense was theorical (n.), late 15c. Sense of "principles or methods of a science or art" (rather than its practice) is first recorded 1610s (as in music theory, which is the science of musical composition, apart from practice or performance). Sense of "an intelligible explanation based on observation and reasoning" is from 1630s.
"move back and forth," Old English wafian "to wave, fluctuate" (related to wæfre "wavering, restless, unstable"), from Proto-Germanic *wab- (source also of Old Norse vafra "to hover about," Middle High German waben "to wave, undulate"), possibly from PIE root *(h)uebh- "to move to and fro; to weave" (see weave (v.)). Transitive sense is from mid-15c.; meaning "to make a sign by a wave of the hand" is from 1510s. Related: Waved; waving.
I was much further out than you thought
And not waving but drowning.
[Stevie Smith]
"moving billow of water," 1520s, alteration (by influence of wave (v.)) of Middle English waw, which is from Old English wagian "to move to and fro," from Proto-Germanic *wag- (source also of Old Saxon, Old High German wag, Old Frisian weg, Old Norse vagr "water in motion, wave, billow," Gothic wegs "tempest"), probably from PIE root *wegh- "to go, move." The usual Old English word for "moving billow of water" was yð.
The "hand motion" meaning is recorded from 1680s; meaning "undulating line" is recorded from 1660s. Of people in masses, first recorded 1852; in physics, from 1832. Sense in heat wave is from 1843. The crowd stunt in stadiums is attested under this name from 1984, the thing itself said to have been done first Oct. 15, 1981, at the Yankees-A's AL championship series game in the Oakland Coliseum; soon picked up and popularized at University of Washington. To make waves "cause trouble" is attested from 1962.
"to touch down," as a bird from flight, "get down or descend," as a person from horseback, from Old English lihtan "to alight; to alleviate, make less heavy," from Proto-Germanic *linkhtijan, literally "to make light," from *lingkhtaz "not heavy" (see light (adj.1)). Apparently the etymological sense is "to dismount" (a horse, etc.), and thus relieve it of one's weight."
Alight has become the more usual word. To light on "happen upon, come upon" is from late 15c. To light out "leave hastily, decamp" is 1866, from a nautical meaning "move out, move heavy objects" (1841), a word of unknown origin but perhaps belonging to this word (compare lighter (n.1)).
"not heavy, having little actual weight," from Old English leoht (West Saxon), leht (Anglian), "not heavy, light in weight; lightly constructed; easy to do, trifling; quick, agile," also of food, sleep, etc., from Proto-Germanic *lingkhtaz (source also of Old Norse lettr, Swedish lätt, Old Frisian, Middle Dutch licht, German leicht, Gothic leihts), from PIE root *legwh- "not heavy, having little weight." The adverb is Old English leohte, from the adjective.
Meaning "frivolous" is from early 13c.; that of "unchaste" from late 14c., both from the notion of "lacking moral gravity" (compare levity). Of literature from 1590s. Light industry (1919) makes use of relatively lightweight materials. The notion in make light of (1520s) is "unimportance." Alternative spelling lite, the darling of advertisers, is first recorded 1962. Light horse "light armed cavalry" is from 1530s. Light-skirts "woman of easy virtue" is attested from 1590s. Lighter-than-air (adj.) is from 1887.
"to shed light; to set on fire," late Old English lihtan (Anglian), liehtan (West Saxon), originally transitive, "to ignite, set on fire," also in a spiritual sense, "to illuminate, fill with brightness." It is common Germanic (cognates: Old Saxon liohtian, Old High German liuhtan, German leuchten, Gothic liuhtjan "to light"), from the source of light (n.).
Meaning "furnish light for" is from c. 1200; sense of "emit light, shed light, shine" is from c. 1300. Buck writes that light is "much more common than kindle even with fire, and only light, not kindle, with candle, lamp, pipe, etc." To light up is from c. 1200 as "give light to" (a room, etc.); 1861 in reference to a pipe, cigar, etc. Related: Lighted; lighting.
"not dark," Old English leoht (West Saxon), leht (Anglian), "luminous, bright, beautiful, shining; having much light," from Proto-Germanic *leuhta- (source also of Old Saxon and Old High German lioht, Old Frisian liacht, German licht "bright"), from the source of Old English leoht (see light (n.)). Meaning "pale-hued" is from 1540s; prefixed to other color adjectives from early 15c. In earlier Middle English in reference to colors it meant "bright, vivid" (early 14c.).
"brightness, radiant energy, that which makes things visible," Old English leht (Anglian), leoht (West Saxon), "light, daylight; spiritual illumination," from Proto-Germanic *leukhtam (source also of Old Saxon lioht, Old Frisian liacht, Middle Dutch lucht, Dutch licht, Old High German lioht, German Licht, Gothic liuhaþ "light"), from PIE root *leuk- "light, brightness."
The -gh- was an Anglo-French scribal attempt to render the Germanic hard -h- sound, which has since disappeared from this word. The figurative spiritual sense was in Old English; the sense of "mental illumination" is first recorded mid-15c. Meaning "something used for igniting" is from 1680s. Meaning "a consideration which puts something in a certain view" (as in in light of) is from 1680s. Short for traffic light from 1938. Quaker use is by 1650s; New Light/Old Light in church doctrine also is from 1650s. Meaning "person eminent or conspicuous" is from 1590s. A source of joy or delight has been the light of (someone's) eyes since Old English:
Ðu eart dohtor min, minra eagna leoht [Juliana].
Phrases such as according to (one's) lights "to the best of one's natural or acquired capacities" preserve an older sense attested from 1520s. To figuratively stand in (someone's) light is from late 14c. To see the light "come into the world" is from 1680s; later as "come to full realization" (1812). The rock concert light-show is from 1966. To be out like a light "suddenly or completely unconscious" is from 1934.
1960, of cinema (from French Nouvelle Vague, late 1950s); 1976 as a name for the more restrained and melodic alternative to punk rock.
