Source : Webster's Revised Unabridged Dictionary (1913)

Abomasum \Ab`o*ma"sum\, Abomasus \Ab`o*ma"sus\, n. [NL., fr. L.
   ab + omasum (a Celtic word).] (Anat.)
   The fourth or digestive stomach of a ruminant, which leads
   from the third stomach omasum. See {Ruminantia}.

Source : WordNet®

abomasum
     n : the fourth compartment of the stomach of a ruminant; the one
         where digestion takes place [syn: {fourth stomach}]