1. (n.) A mollusk having a shell consisting of two lateral plates or valves joined together by an elastic ligament at the hinge, which is usually strengthened by prominences called teeth. The shell is closed by the contraction of two transverse muscles attached to the inner surface, as in the clam, -- or by one, as in the oyster. See Mollusca.

2. (n.) A pericarp in which the seed case opens or splits into two parts or valves.

3. (a.) Having two shells or valves which open and shut, as the oyster and certain seed vessels.