Source : Webster's Revised Unabridged Dictionary (1913)

Omasum \O*ma"sum\, n. [L.] (Anat.)
   The third division of the stomach of ruminants. See
   {Manyplies}, and Illust. under {Ruminant}.

Source : WordNet®

omasum
     n : the third compartment of the stomach of a ruminant [syn: {psalterium},
          {third stomach}]
     [also: {omasa} (pl)]