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.