valve

noun

• a device for controlling the passage of fluid through a pipe or duct, especially an automatic device allowing movement in one direction only.
• (in full thermionic valve) ElectronicsBritish term for thermionic tube.
• 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.