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 megas "great, large, vast, big, high, tall; mighty, important," from PIE root *meg- "great." The final letter of the Greek alphabet, hence used figuratively for "the last, final" of anything (as in Revelations i.8).