valve

1 Any of various structures for restricting the flow of a fluid through an aperture or along a tube to one direction. Valves in the heart (see bicuspid valve; semilunar valve; tricuspid valve), veins, and lymphatic vessels consist of two or three flaps of tissue (cusps) fastened to the walls. The cusps are flattened to the walls to allow the normal passage of blood or lymph, but a reverse flow causes them to block the vessel or aperture, so preventing further backflow.

2 Any of the parts that make up a capsule or other dry fruit that sheds its seeds.

3 One of the two halves of the cell wall of a diatom.

4 Either of the two hinged portions of the shell of a bivalve mollusc.