omega (n.)

final letter of the Greek alphabet, 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 o + megas "great, large, vast, big, high, tall; mighty, important," from PIE root *meg- "great." Used figuratively for "the last, the final" of anything (as in Revelation i.8) from 1520s.