n 1  :  either of two valves of which one is situated at the opening between the heart and the aorta and the other at the opening between the heart and the pulmonary artery, which prevent regurgitation of blood into the ventricles, and each of which is made up of three crescent-shaped cusps that are forced apart by pressure in the ventricles exerted during systole and are pushed together by pressure in the arteries exerted during diastole  2  :  SEMILUNAR CUSP .
 
 
 
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