What does "semilunar" mean?

Semilunar: 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.