omicron

[om-i-kron, oh-mi-] /ˈɒm ɪˌkrɒn, ˈoʊ mɪ-/
noun
1.
the 15th letter of the Greek alphabet (O, o).
2.
the vowel sound represented by this letter.
Origin
< Greek ō mikrón, literally, small o. Cf. omega
British Dictionary definitions for omicron

omicron

/əʊˈmaɪkrɒn; ˈɒmɪkrɒn/
noun
1.
the 15th letter in the Greek alphabet (Ο, ο), a short vowel, transliterated as o
Word Origin
from Greek ō mikron small o; see micro-, omega
Word Origin and History for omicron

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 in Medicine

omicron om·i·cron (ŏm'ĭ-krŏn', ō'mĭ-)
n.

Symbol o The 15th letter of the Greek alphabet.