"one millionth of a meter," by 1883, coined in French from Greek mikron, neuter of mikros "small" (see micro-) and formally adopted Oct. 2, 1879, by the Comité International des Poids et Mesures. It was replaced 1968 by the micrometre.
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.