15th letter of the Greek alphabet, literally "small 'o,' " from Greek (s)mikros "small," from PIE *smik-. Because the vowel was "short" in ancient Greek. Cf. Omega.
omicron om·i·cron (ŏm'ĭ-krŏn', ō'mĭ-)n. Symbol o The 15th letter of the Greek alphabet.