Etymology
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Words related to *leuk-

allumette (n.)

"match for lighting," 1848, from French allumette "a match," from allumer "to light, kindle," from Old French alluminer, from Latin ad "to" (see ad-) + luminare "to shine," from Latin lumen (genitive luminis) "light" (from suffixed form of PIE root *leuk- "light, brightness").

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elucidate (v.)

"make clear or manifest, throw light upon, explain, render intelligible," 1560s, perhaps via French élucider (15c.) or directly from Late Latin elucidatus, past participle of elucidare "make light or clear," from assimilated form of ex "out, away" (see ex-) + lucidus "light, bright, clear," figuratively "perspicuous, lucid, clear," from lucere "to shine" (from PIE root *leuk- "to shine, be bright"). Related: Elucidated; elucidates; elucidating.

illumination (n.)
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.
illustration (n.)
c. 1400, "a shining;" early 15c., "a manifestation;" mid-15c., "a spiritual illumination," from Old French illustration "apparition, appearance" (13c.) and directly from Latin illustrationem (nominative illustratio) "vivid representation" (in writing), literally "an enlightening," from past participle stem of illustrare "light up, make light, illuminate;" figuratively "make clear, disclose, explain; adorn, render distinguished," from assimilated form of in- "in" (from PIE root *en "in") + lustrare "make bright, illuminate," from suffixed form of PIE root *leuk- "light, brightness." Mental sense of "act of making clear in the mind" is from 1580s. Meaning "an illustrative picture" is from 1816.
lea (n.)
Old English leah "open field, meadow, piece of untilled grassy ground," earlier læch, preserved in place names, from Proto-Germanic *lauhaz (source also of Old High German loh "clearing," and probably also Flemish -loo, which forms the second element in Waterloo), from PIE *louko- "light place" (source also of Sanskrit lokah "open space, free space, world," Latin lucus "grove, sacred grove, wood," Lithuanian laukas "open field, land"), from root *leuk- "to shine, be bright." The dative form is the source of many of the English surnames Lee, Leigh.
leukemia (n.)
progressive blood disease characterized by abnormal accumulation of leucocytes, 1851, on model of German Leukämie (1848), coined by R. Virchow from Greek leukos "clear, white" (from PIE root *leuk- "light, brightness") + haima "blood" (see -emia). Formerly also leucemia.
leuko- 
before vowels leuk-, also sometimes in Latinized form leuco-/leuc-, word-forming element used from 19c. and meaning "white" (or, in medicine, "leukocyte"), from Greek leukos "clear, white," from PIE *leuko-, suffixed form of root *leuk- "light, brightness."
light (n.)

"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.

lightning (n.)
visible discharge of energy between cloud and cloud or cloud and ground, late Old English, "lightning, flash of lightning," verbal noun from lightnen "make bright," or else an extended form of Old English lihting, from leht (see light (n.)). The Old English word also meant "dawn, daybreak," and in Middle English "light of the sun, intense brightness, brilliance; the radiance of Christ." Another Middle English word for it was leven (mid-13c.), of uncertain origin, with no apparent source in Old English. (Old English had ligetung "lightning," from liget "lightning, flash of lightning." "Lightning" also was a specialized sense of lihting "lighting" and beorhtnes "brightness.")

Meaning "cheap, raw whiskey" is attested from 1781, also sometimes "gin." Lightning bug "firefly, phosphorescent beetle" is attested from 1778. Lightning rod from 1790.
limn (v.)
early 15c., "to illuminate" (manuscripts), altered from Middle English luminen, "to illuminate manuscripts" (late 14c.), from Old French luminer "light up, illuminate," from Latin luminare "illuminate, burnish," from lumen (genitive luminis) "radiant energy, light," related to lucere "to shine," from PIE *leuk-smen-, suffixed form of root *leuk- "light, brightness." Figurative sense of "portray, depict" first recorded 1590s. Related: Limned; limner.