What is the X in X-mas?
c.1400, from Medieval Greek omega, from classical Greek o mega "big 'o' " (in contrast to o micron "little 'o' "); so called because the vowel was long in ancient Greek. From mega (see mega-). The final letter of the Greek alphabet, hence used figuratively for "the last, final" of anything (cf. Rev. i:8),
omega o·me·ga (ō-měg'ə, ō-mē'gə, ō-mā'-)
Symbol ω, Ω The 24th letter of the Greek alphabet. adj.
Of or characterizing a chemical group or position at the end of a molecular chain, such as omega-oxidation.
(Rev. 1:8), the last letter in the Greek alphabet. (See A.)