The Collaborative International Dictionary of English v.0.48:

Post-abdomen \Post`-ab*do"men\, n. [Pref. post- + abdomen.]
   (Zool.)
   That part of a crustacean behind the cephalothorax; -- more
   commonly called abdomen.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Abdomen \Ab*do"men\, n. [L. abdomen (a word of uncertain
   etymol.): cf. F. abdomen.]
   1. (Anat.) The belly, or that part of the body between the
      thorax and the pelvis. Also, the cavity of the belly,
      which is lined by the peritoneum, and contains the
      stomach, bowels, and other viscera. In man, often
      restricted to the part between the diaphragm and the
      commencement of the pelvis, the remainder being called the
      pelvic cavity.
      [1913 Webster]

   2. (Zool.) The posterior section of the body, behind the
      thorax, in insects, crustaceans, and other Arthropoda.
      [1913 Webster]

WordNet (r) 3.0 (2006):

abdomen
    n 1: the region of the body of a vertebrate between the thorax
         and the pelvis [syn: abdomen, venter, stomach,
         belly]
    2: the cavity containing the major viscera; in mammals it is
       separated from the thorax by the diaphragm [syn: abdominal
       cavity, abdomen]

The Devil's Dictionary (1881-1906):

ABDOMEN, n.  The temple of the god Stomach, in whose worship, with
sacrificial rights, all true men engage.  From women this ancient
faith commands but a stammering assent.  They sometimes minister at
the altar in a half-hearted and ineffective way, but true reverence
for the one deity that men really adore they know not.  If woman had a
free hand in the world's marketing the race would become
graminivorous.