The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

abomasum
    n 1: the fourth compartment of the stomach of a ruminant; the
         one where digestion takes place [syn: abomasum, fourth
         stomach]