The Collaborative International Dictionary of English v.0.48:

Sigmoid \Sig"moid\, Sigmoidal \Sig*moid"al\, a. [Gr. ???; ???
   sigma + e'i^dos form, likeness: cf. F. sigmo["i]de.]
   Curved in two directions, like the letter S, or the Greek
   [sigmat].
   [1913 Webster]

   Sigmoid flexure (Anat.), the last curve of the colon before
      it terminates in the rectum. See Illust. under
      Digestive.

   Sigmoid valves. (Anat.) See Semilunar valves, under
      Semilunar.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sigmoid flexure
    n 1: the s-shaped curve between the descending colon and the
         rectum [syn: sigmoid colon, sigmoid flexure]