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]