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