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Medical Definition of Semilunar cusp
1. One of the three semilunar segments serving as the three cusps of a valve preventing regurgitation at the beginning of the aorta; a similar valve guards the entrance of the pulmonary trunk; the segments are named, respectively, anterior, right, and left in the pulmonary valve, and posterior, right, and left in the aortic valve. (05 Mar 2000)