The Collaborative International Dictionary of English v.0.48:

Semilunar \Sem`i*lu"nar\, a.
   Shaped like a half moon.
   [1913 Webster]

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

   Semilunar valves, 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.
      [1913 Webster]