/ Dictionary / Index O omasum: Meaning and Definition of Find definitions for: o•ma•sum Pronunciation: (ō-mā'sum), [key] — pl. -sa the third stomach of a ruminant, between the reticulum and the abomasum; the manyplies. See diag. under Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: omasum (Thesaurus) Omar Khayyám Omayyad Related Content Daily Word Quiz: reclusive Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language