a door; especially, one of a pair of folding doors, or one of the leaves of such a door.-a lid, plug, or cover, applied to an aperture so that by its movement, as by swinging, lifting and falling, sliding, turning, or the like, it will open or close the aperture to permit or prevent passage, as of a fluid.-one or more membranous partitions, flaps, or folds, which permit the passage of the contents of a vessel or cavity in one direction, but stop or retard the flow in the opposite direction; as, the ileocolic, mitral, and semilunar valves.-one of the pieces into which a capsule naturally separates when it bursts.-one of the two similar portions of the shell of a diatom.-a small portion of certain anthers, which opens like a trapdoor to allow the pollen to escape, as in the barberry.-one of the pieces or divisions of bivalve or multivalve shells.