The Collaborative International Dictionary of English v.0.48:

Rectum \Rec"tum\ (-t?m), n. [NL. (sc. intestinum), fr. L. rectus
   straight. See Right.] (Anat.)
   The terminal part of the large intestine; -- so named because
   supposed by the old anatomists to be straight. See Illust.
   under Digestive.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rectum
    n 1: the terminal section of the alimentary canal; from the
         sigmoid flexure to the anus

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "rectum":
   abdomen, anus, appendix, blind gut, bowels, brain, cecum, colon,
   duodenum, endocardium, entrails, foregut, giblets, gizzard, guts,
   heart, hindgut, innards, inner mechanism, insides, internals,
   intestine, inwards, jejunum, kidney, kishkes, large intestine,
   liver, liver and lights, lung, midgut, perineum, pump, pylorus,
   small intestine, spleen, stomach, ticker, tripes,
   vermiform appendix, viscera, vitals, works