What is the X in X-mas?
Old English candel "lamp, lantern, candle," an early ecclesiastical borrowing from Latin candela "a light, torch, candle made of tallow or wax," from candere "to shine," from PIE root *kand- "to glow, to shine, to shoot out light" (cf. Sanskrit cand- "to give light, shine," candra- "shining, glowing, moon;" Greek kandaros "coal;" Welsh cann "white;" Middle Irish condud "fuel").
Candles were unknown in ancient Greece (where oil lamps sufficed), but common from early times among Romans and Etruscans. Candles on birthday cakes seems to have been originally a German custom. To hold a candle to originally meant "to help in a subordinate capacity," from the notion of an assistant or apprentice holding a candle for light while the master works. To burn the candle at both ends is recorded from 1730.
candle can·dle (kān'dl)
Heb. ner, Job 18:6; 29:3; Ps. 18:28; Prov. 24:20, in all which places the Revised Version and margin of Authorized Version have "lamp," by which the word is elsewhere frequently rendered. The Hebrew word denotes properly any kind of candle or lamp or torch. It is used as a figure of conscience (Prov. 20:27), of a Christian example (Matt. 5:14, 15), and of prosperity (Job 21:17; Prov. 13:9).
light source now mostly used for decorative and ceremonial purposes, consisting of wax, tallow, or similar slow-burning material, commonly in cylindrical form but made in many fanciful designs, enclosing and saturating a fibrous wick.