Either of two valves in the heart, found in the pulmonary artery (pulmonary valve) and in the aorta (aortic valve), that prevent the backflow of blood into the right and left ventricles from the pulmonary artery and the aorta, respectively, thus maintaining blood flow in a single direction. Each semilunar valve consists of three pockets, which close by filling with blood when the force of contraction that expels the blood from the heart diminishes.