1a device for controlling the passage of fluid through a pipe or duct, 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 which, 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, which maintains the flow of the contents in one direction by closing in response to any pressure from reverse flow:the aortic valve
2 Zoology each of the halves of the hinged shell of a bivalve mollusc 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.
[in combination]:a branchiopod has a two-valved outer covering
late Middle English (denoting a leaf of a folding or double door): from Latin valva