Source : Webster's Revised Unabridged Dictionary (1913)

Semilunar \Sem`i*lu"nar\, a.
   Shaped like a half moon.

   {Semilunar bone} (Anat.), a bone of the carpus; the lunar.
      See {Lunar}, n.

   {Semilunar}, or {Sigmoid}, {valves} (Anat.), the valves at
      the beginning of the aorta and of the pulmonary artery
      which prevent the blood from flowing back into the
      ventricle.

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

   {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}.

Source : WordNet®

sigmoid
     adj 1: curved in two directions (like the letter S)
     2: of or relating to the sigmoid flexure in the large intestine
        [syn: {sigmoidal}]