gel-

Root for cold/freezing, yielding Latin gelu (frost), English cold, cool, chill.‌​‌​‌​‌​‍​‍​‍​‍​‍​‌​‍​‌​‌​‍​‍​‌​‌​‌​‌​‌​‌​‌​‌​‍​‌​‌​‌​‍​‍​‌​‍​‌

Discussion

The PIE root *lewk- (light, brightness, to shine) produced one of the largest and most culturally si‌​‌​‌​‌​‍​‍​‍​‍​‍​‌​‍​‌​‌​‍​‍​‌​‌​‌​‌​‌​‌​‌​‌​‍​‌​‌​‌​‍​‍​‌​‍​‌gnificant word families in the Indo-European vocabulary — a set of descendants that spans celestial bodies, visual perception, intellectual clarity, and the technology of illumination.

Latin lūx (light — genitive lūcis) is the primary Italic reflex and the source of an extraordinary English vocabulary: lucid (full of light, clear), elucidate (to bring light to, to clarify), translucent (shining through), and Lucifer (light-bearer — the morning star, later applied to the fallen angel). The related Latin verb lūcēre (to shine) produced lucent, and the diminutive lūcerna (lamp) named the lighting device.

The derivative lūna (moon — the shining one) extended the root to astronomy: lunar, lunatic (moon-struck — the ancient belief that the full moon caused madness), lunation, and sublunary (beneath the moon, earthly). The moon's name as "the shining one" parallels the sun's name from a different root (*sóh₂wl̥), showing that PIE speakers named celestial bodies by their luminous properties.

Latin illustrāre (to light up, to make clear, to adorn with pictures — from in- + lūstrāre) gave English illustrate, illustration, and illustrious (shining with fame). The semantic chain light → clarity → fame → pictures encodes a conceptual progression in which illumination is the master metaphor.

Greek leukós (λευκός, "white, bright, shining") preserves the root in the chromatic register: leukocyte (white blood cell), leukemia (white blood disease — named for the pale appearance of affected blood), and the name Luke/Lucia/Lucian (all "light-names").

The Germanic branch shows the root most transparently in English light (OE lēoht, from PGmc *leuhtą). German Licht, Dutch licht, and the Scandinavian forms continue the same word. The verb light (to illuminate, OE līhtan) and the adjective light (not dark) both descend from this root, though the adjective light (not heavy) is from a different PIE source (*h₁lengʷʰ-).

Sanskrit rokás (light) and the related rúci- (brightness, beauty) provide Indo-Iranian attestation, with the expected satem treatment of the initial consonant cluster. Lithuanian laũkas (pale, whitish) confirms the Baltic reflex.

The root's survival across every major branch — with the same core meaning of luminosity — makes it one of the most securely reconstructed PIE items. That it generated vocabularies for astronomy, medicine, philosophy, art, and theology reflects the primacy of light as the organising metaphor of IE thought: to see is to know, to shine is to excel, and what is dark is what is unknown.

Notes

Pokorny 365-366. English glacier, gelatin, congeal.