A valve is a mechanical device that regulates the flow of fluids (either gases and liquids) by opening, closing, or partially obstructing various passageways.

There are a number of names for particular type of valve:

  • Cock, colloqual term for a small valve
  • Tap (British english), Faucet (US english)
  • Bib cock,
  • Non-return valve, allows the fluid to pas in one direction only
  • Ball cock, Often used as a water level controller (cistern)
  • Solenoid valve, electrically controlled

A valve doesn't necessarily mean a man-made device; there are several valves inside the human body, for instance in the heart or in veins.

Because their effect on a flow of electrons is analogous to the fluid devices, the electronic devices known in the US as "vacuum tubes" are called "thermionic valves" in Britain.