omasum

from WordNet (r) 3.0 (2006)

omasum
    n 1: the third compartment of the stomach of a ruminant [syn:
         {psalterium}, {omasum}, {third stomach}]

from The Collaborative International Dictionary of English v.0.48

Omasum \O*ma"sum\, n. [L.] (Anat.)
   The third division of the stomach of ruminants. See
   {Manyplies}, and Illust. under {Ruminant}.
   [1913 Webster] Omber