English▸ noun a device for controlling the passage of fluid or air through a pipe, duct, etc., especially an automatic device allowing movement in one direction only:
a valve shuts off the flow from the boiler when the water is hot enough.
▪ Music a cylindrical mechanism in a brass instrument that, when depressed or turned, admits air into different sections of tubing and so extends the range of available notes.
▪ Anatomy & Zoology a membranous fold in a hollow organ or tubular structure, such as a blood vessel or the digestive tract, that maintains the flow of the contents in one direction by closing in response to any pressure from reverse flow.
▪ Zoology each of the halves of the hinged shell of a bivalve mollusk or brachiopod, or of the parts of the compound shell of a barnacle.
▪ Botany each of the halves or sections into which a dry fruit (especially a pod or capsule) dehisces.
– ORIGIN late Middle English (denoting a leaf of a folding or double door): from Latin valva.