Definition of semilunar valve
: either of two heart valves of which one occurs between the heart and the aorta and the other between the heart and pulmonary artery and that consist of three crescent-shaped flaps that are forced apart by pressure in the ventricles during systole and pushed together by pressure in the arteries during diastole and which prevent regurgitation of blood into the ventricles;
also
: any of the crescent-shaped flaps making up the semilunar valves
