What is the X in X-mas?
"brightness, radiant energy," Old English leht, earlier leoht "light, daylight; luminous, beautiful," from West Germanic *leukhtam (cf. Old Saxon lioht, Old Frisian liacht, Middle Dutch lucht, Dutch licht, Old High German lioht, German Licht, Gothic liuhaþ "light"), from PIE *leuk- "light, brightness" (cf. 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").
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 (e.g. in light of) is from 1680s. Something that's a joy and a delight has been the light of (someone's) eyes since Old English:
Ðu eart dohtor min, minra eagna leoht [Juliana].To see the light "come into the world" is from 1680s; later in a Christian sense.
"not heavy," from Old English leoht "not heavy, light in weight; easy, trifling; quick, agile," from Proto-Germanic *lingkhtaz (cf. Old Norse lettr, Swedish lätt, Old Frisian, Middle Dutch licht, German leicht, Gothic leihts), from PIE root *legwh- "not heavy, having little weight" (cf. Latin levis "light," Old Irish lu "small;" see lever).
The notion in make light of (1520s) is of "unimportance." Alternative spelling lite, the darling of advertisers, is first recorded 1962. The adverb is Old English leohte, from the adjective. Light-skirts "woman of easy virtue" is attested from 1590s. To make light of is from 1520s.
"not dark," Old English leoht, common Germanic (cf. Old Saxon and Old High German lioht, Old Frisian liacht, German licht "bright," from the source of Old English leoht (see light (n.)). Meaning "pale-hued" is from 1540s.
"touch down," from Old English lihtan "to alight; alleviate, leave," from Proto-Germanic *linkhtijan, literally "to make light," from *lingkhtaz "not heavy" (see light (adj.1)). Apparently the ground sense is "to dismount a horse, etc., and thus relieve it of one's weight." To light out "leave hastily" is 1870, from a nautical meaning "move out, move heavy objects," of unknown origin but perhaps belonging to this word (cf. lighter (n.1)).
Electromagnetic radiation that has a wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be perceived by the normal unaided human eye.
Electromagnetic radiation of any wavelength.
the offspring of the divine command (Gen. 1:3). "All the more joyous emotions of the mind, all the pleasing sensations of the frame, all the happy hours of domestic intercourse were habitually described among the Hebrews under imagery derived from light" (1 Kings 11:36; Isa. 58:8; Esther 8:16; Ps. 97:11). Light came also naturally to typify true religion and the felicity it imparts (Ps. 119:105; Isa. 8:20; Matt. 4:16, etc.), and the glorious inheritance of the redeemed (Col. 1:12; Rev. 21:23-25). God is said to dwell in light inaccessible (1 Tim. 6:16). It frequently signifies instruction (Matt. 5:16; John 5:35). In its highest sense it is applied to Christ as the "Sun of righteousness" (Mal. 4:2; Luke 2:32; John 1:7-9). God is styled "the Father of lights" (James 1:17). It is used of angels (2 Cor. 11:14), and of John the Baptist, who was a "burning and a shining light" (John 5:35), and of all true disciples, who are styled "the light of the world" (Matt. 5:14).