What is the X in X-mas?
early 13c., from Old French mireoir "a reflecting glass, looking glass; observation, model, example," earlier miradoir (11c.), from mirer "look at" (oneself in a mirror), "observe, watch, contemplate," from Vulgar Latin *mirare "to look at," variant of Latin mirari "to wonder at, admire" (see miracle). Figurative usage is attested from c.1300. Used in divination since classical and biblical times; mirrors in modern England are the subject of at least 14 known superstitions, according to folklorists. Belief that breaking one brings bad luck is attested from 1777. The Spanish cognate, mirador (from mirar "to look, look at, behold"), has come to mean "watch tower." Mirror ball attested from 1968.
"to reflect," 1590s, from mirror (n.). Related: Mirrored; mirroring. The Middle English verb mirouren (early 15c.) meant "to be a model" (for conduct, behavior, etc.), while miren (mid-14c., from Old French mirer) meant "to look in a mirror."
An object that causes light or other radiation to be reflected from its surface, with little or no diffusion. Common mirrors consist of a thin sheet or film of metal, such as silver, behind or covering a glass pane. Mirrors are used extensively in telescopes, microscopes, lasers, fiber optics, measuring instruments, and many other devices. See more at reflection.