late 14c., "spiritual enlightenment," from Late Latin illuminationem (nominative illuminatio), noun of action from past participle stem of Latin illuminare "to throw into light, make bright, light up;" figuratively, in rhetoric, "to set off, illustrate," from assimilated form of in- "in, into" (from PIE root *en "in") + lumen (genitive luminis) "light," from suffixed form of PIE root *leuk- "light, brightness." Meaning "action of lighting" in English is from 1560s; sense of "intellectual enlightenment" is from 1630s.

late 14c., "to enlighten spiritually;" mid-15c., "to light up, shine light on," from Old French illuminer (13c.), from Latin illuminare "make bright, light up" (see illumination). Related: illumined.

"illuminate," c. 1600, from French illumer, contraction of illuminer, from Latin illuminare "light up, make light, illuminate" (see illumination). Related: Illumined; illumining.

c. 1500, "to light up, shine on," a back-formation from illumination or else from Latin illuminatus, past participle of illuminare "light up, make light, illuminate." Earlier was enlumyen (late 14c.) "decorate written material by hand with gold, silver, or bright colors," from Old French enluminer, from Late Latin inluminare; also illumine (late 14c.). Related: Illuminated; illuminating; illuminable.

1590s, plural of Latin illuminatus "enlightened" (in figurative sense), past participle of illuminare "light up, make light, illuminate" (see illumination). Originally a name applied to a 16c. Spanish sect (the Alumbrados), then to other sects on the continent; since 1797 used as a translation of German Illuminaten, name of a secret society founded 1776 in Ingolstadt, Bavaria, (repressed there 1785) and holding deistic and republican principles; hence used generally of free-thinkers and sarcastically of those professing intellectual enlightenment (1816). Related: Illuminatism; illuminatist.

"shining, illumination," Old English lihting "shining, illumination; dawn; lightning," from leoht (see light (n.)).

unit of illumination, 1889, from Latin lux "light," from PIE root *leuk- "light, brightness."

electric lighting unit, 1921, a trade term, from French luminaire, from Old French luminarie "lamp, candle; brightness, illumination" (see luminary).

"stimulation or inhibition to breeding, etc., based on daily periods of light and darkness," 1920, from photoperiod "period of daily illumination which an organism receives" (1920, from photo- + period) + -ism.

also flash-light, 1886, "on-and-off signal light in a light-house, etc.," from flash (v.) + light (n.). As the word for a photographer's light-emitting preparation, 1892 (flash-lamp in this sense is by 1890). From 1905 as as a handheld, pocket-sized electric illumination device, the American English word for what the British might call an electric torch.