Etymology
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*leuk- 

Proto-Indo-European root meaning "light, brightness."

It forms all or part of: allumette; elucidate; illumination; illustration; lea; leukemia; leuko-; light (n.) "brightness, radiant energy;" lightning; limn; link (n.2) "torch of pitch, tow, etc.;" lucent; lucid; Lucifer; luciferase; luciferous; lucifugous; lucubrate; lucubration; luculent; lumen; Luminal; luminary; luminate; luminescence; luminous; luna; lunacy; lunar; Lunarian; lunate; lunation; lunatic; lune; lunette; luni-; luster; lustrum; lux; pellucid; sublunary; translucent.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit rocate "shines;" Armenian lois "light," lusin "moon;" Greek leukos "bright, shining, white;" Latin lucere "to shine," lux "light," lucidus "clear;" Old Church Slavonic luci "light;" Lithuanian laukas "pale;" Welsh llug "gleam, glimmer;" Old Irish loche "lightning," luchair "brightness;" Hittite lukezi "is bright;" Old English leht, leoht "light, daylight; spiritual illumination," German Licht, Gothic liuhaþ "light."

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*legwh- 
Proto-Indo-European root meaning "not heavy, having little weight."

It forms all or part of: alleviate; alleviation; alto-rilievo; carnival; elevate; elevation; elevator; leaven; legerdemain; leprechaun; Levant; levator; levee; lever; levity; levy (v.) "to raise or collect;" light (adj.1) "not heavy, having little weight;" lighter (n.1) "type of barge used in unloading;" lung; relevance; relevant; releve; relief; relieve.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit laghuh "quick, small;" Greek elakhys "small," elaphros "light;" Latin levare "to raise," levis "light in weight, not heavy;" Old Church Slavonic liguku, Russian lëgkij, Polish lekki, Lithuanian lengvas "light in weight;" Old Irish lu "small," laigiu "smaller, worse;" Gothic leihts, Old English leoht "not heavy, light in weight."
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*bha- (1)

*bhā-, Proto-Indo-European root meaning "to shine."

It forms all or part of: aphotic; bandolier; banner; banneret; beacon; beckon; buoy; diaphanous; emphasis; epiphany; fantasia; fantasy; hierophant; pant (v.); -phane; phanero-; phantasm; phantasmagoria; phantom; phase; phene; phenetic; pheno-; phenology; phenomenon; phenyl; photic; photo-; photocopy; photogenic; photograph; photon; photosynthesis; phosphorus; phaeton; sycophant; theophany; tiffany; tryptophan.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhati "shines, glitters;" Greek phainein "bring to light, make appear," phantazein "make visible, display;" Old Irish ban "white, light, ray of light."

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*aug- (1)

Proto-Indo-European root meaning "to increase." It forms all or part of: auction; augment; augmentative; augur; August; august; Augustus; author; authoritarian; authorize; auxiliary; auxin; eke (v.); inaugurate; nickname; waist; wax (v.1) "grow bigger or greater."

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit ojas- "strength," vaksayati "cause to grow;" Lithuanian augu, augti "to grow," aukštas "high, of superior rank;" Greek auxo "increase," auxein "to increase;" Gothic aukan "to grow, increase;" Latin augmentum "an increase, growth," augere "to increase, make big, enlarge, enrich;" Old English eacien "to increase," German wachsen, Gothic wahsjan "to grow, increase."

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*kand- 
also *kend-, Proto-Indo-European root meaning "to shine."

It forms all or part of: candela; candelabrum; candescent; candid; candidate; candle; candor; chandelier; chandler; frankincense; incandescence; incandescent; incendiary; incense (n.) "substance producing a sweet smell when burned;" incense (v.1) "to provoke, anger."

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit cand- "to give light, shine," candra- "shining, glowing, moon;" Greek kandaros "coal;" Latin candere "to shine;" Welsh cann "white," Middle Irish condud "fuel."
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*reudh- 
Proto-Indo-European root meaning "red, ruddy." The only color for which a definite common PIE root word has been found. The initial -e- in the Greek word is because Greek tends to avoid beginning words with -r-.

It forms all or part of: bilirubin; corroborate; Eritrea; erysipelas; erythema; erythro-; Radnor; red; redskin; roan; robust; rooibos; Rotwelsch; rouge; roux; rowan; rubella; rubicund; rubric; ruby; ruddock; ruddy; rufous; Rufus; russet; rust.

It is the hypothetical source of/evidence for its existence is provided by: Latin ruber, also dialectal rufus "light red," mostly of hair; Greek erythros; Sanskrit rudhira-; Avestan raoidita-; Old Church Slavonic rudru, Polish rumiany, Russian rumjanyj "flushed, red," of complexions, etc.; Lithuanian raudas; Old Irish ruad, Welsh rhudd, Breton ruz "red."
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*sawel- 

*sāwel-, Proto-Indo-European root meaning "the sun." According to Watkins, the *-el- in it originally was a suffix, and there was an alternative form *s(u)wen-, with suffix *-en-, hence the two forms represented by Latin sol, English sun.

It forms all or part of: anthelion; aphelion; girasole; heliacal; helio-; heliotrope; helium; insolate; insolation; parasol; parhelion; perihelion; Sol; solar; solarium; solstice; south; southern; sun; Sunday.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit suryah, Avestan hvar "sun, light, heavens;" Greek hēlios; Latin sol "the sun, sunlight;" Lithuanian saulė, Old Church Slavonic slunice; Gothic sauil, Old English sol "sun;" Old English swegl "sky, heavens, the sun;" Welsh haul, Old Cornish heuul, Breton heol "sun;" Old Irish suil "eye;" Avestan xueng "sun;" Old Irish fur-sunnud "lighting up;" Old English sunne German Sonne, Gothic sunno "the sun."

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*ghel- (2)

Proto-Indo-European root meaning "to shine;" it forms words for "gold" (the "bright" metal), words denoting colors, especially "yellow" and "green," also "bile, gall," for its color, and a large group of Germanic gl- words having to do with shining and glittering and, perhaps, sliding. Buck says the interchange of words for yellow and green is "perhaps because they were applied to vegetation like grass, cereals, etc., which changed from green to yellow."

It forms all or part of: arsenic; Chloe; chloral; chloride; chlorinate; chlorine; chloro-; chloroform; chlorophyll; chloroplast; cholecyst; choler; cholera; choleric; cholesterol; cholinergic; Cloris; gall (n.1) "bile, liver secretion;" gild; glad; glance; glare; glass; glaze; glazier; gleam; glee; glib; glide; glimmer; glimpse; glint; glissade; glisten; glister; glitch; glitter; glitzy; gloaming; gloat; gloss (n.1) "glistening smoothness, luster;" glow; glower; gold; guilder; jaundice; melancholic; melancholy; yellow; zloty.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit harih "yellow, tawny yellow," hiranyam "gold;" Avestan zari "yellow;" Old Persian daraniya-, Avestan zaranya- "gold;"  Greek khlōros "greenish-yellow color,"  kholos "bile, gall, wrath;"  Latin helvus "yellowish, bay," Gallo-Latin gilvus "light bay;" Lithuanian geltonas "yellow;" Old Church Slavonic zlutu, Polish żółty, Russian zeltyj "yellow;" Latin galbus "greenish-yellow," fellis "bile, gall;" Lithuanian žalias "green," želvas "greenish," tulžis "bile;" Old Church Slavonic zelenu, Polish zielony, Russian zelenyj "green;" Old Irish glass, Welsh and Breton glas "green," also "gray, blue;" Old English galla "gall, bile," geolu, geolwe, German gelb, Old Norse gulr "yellow;" Old Church Slavonic zlato, Russian zoloto, Old English gold, Gothic gulþ "gold;" Old English glæs "glass; a glass vessel."

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