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]