Source : Webster's Revised Unabridged Dictionary (1913)

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}.

Source : WordNet®

rectum
     n : the terminal section of the alimentary canal; from the
         sigmoid flexure to the anus
     [also: {recta} (pl)]