ETYM Latin valva the leaf, fold, or valve of a door: cf. French valve.
1. A structure in a hollow organ (like the heart) with a flap to insure one-way flow of fluid through it.
2. Any mechanical device for controlling the flow of a fluid.
3. Device in a brass wind instrument for varying the length of the air column to alter the pitch of a tone.

See electron tube, vacuum tube.
(electronics) or electron tube; In electronics, a glass tube containing gas at low pressure, which is used to control the flow of electricity in a circuit.
Three or more metal electrodes are inset into the tube. By varying the voltage on one of them, called the grid electrode, the current through the valve can be controlled, and the valve can act as an amplifier. Valves have been replaced for most applications by transistors.
However, they are still used in high-power transmitters and amplifiers, and in some hi-fi systems.

Device that controls the flow of a fluid. Inside a valve, a plug moves to widen or close the opening through which the fluid passes. The valve was invented by US radio engineer Lee de Forest (1873–1961).
Common valves include the cone or needle valve, the globe valve, and butterfly valve, all named for the shape of the plug. Specialized valves include the one-way valve, which permits fluid flow in one direction only, and the safety valve, which cuts off flow under certain conditions.