Source : Webster's Revised Unabridged Dictionary (1913)

Omicron \O*mi"cron\, n. [Written also {omikron}.] [NL., fr. Gr.
   [Omicron] [omicron]. See {Micro-}.]
   Lit., the little, or short, O, o; the fifteenth letter of the
   Greek alphabet.

Source : WordNet®

omicron
     n : the 15th letter of the Greek alphabet