late 14c., "one of the halves of a folding door," from Latin valva "section of a folding or revolving door," literally "that which turns," related to volvere "to roll" (see volvox). Sense extended 1610s to "membranous fold regulating flow of bodily fluids;" 1650s to "mechanical device that works like a valve;" and 1660s in zoology to "halves of a hinged shell."
A membranous structure in a hollow organ or passage, as in an artery or a vein, that folds or closes to prevent the return flow of the body fluid passing through it.
Any of various devices that regulate the flow of gases, liquids, or loose materials through piping or through apertures by opening, closing, or obstructing ports or passageways.
The movable control element of such a device.