The Collaborative International Dictionary of English v.0.48:

Rumen \Ru"men\, n. [L. rumen, - inis, the throat.]
   1. (Anat.) The first stomach of ruminants; the paunch; the
      fardingbag. See Illust. below.
      [1913 Webster]

   2. The cud of a ruminant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rumen
    n 1: the first compartment of the stomach of a ruminant; here
         food is collected and returned to the mouth as cud for
         chewing [syn: rumen, first stomach]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "rumen":
   abdomen, abomasum, bay window, beerbelly, belly, breadbasket, craw,
   crop, diaphragm, embonpoint, first stomach, gizzard, gullet, gut,
   honeycomb stomach, kishkes, manyplies, maw, midriff, omasum,
   paunch, pot, potbelly, potgut, psalterium, pusgut, rennet bag,
   reticulum, second stomach, spare tire, stomach, swagbelly,
   third stomach, tum-tum, tummy, underbelly, ventripotence