1. A control mechanism or structure that controls the flow of water (or another fluid), gas, etc. :"A valve permits fluid or semifluid material to go in one direction through a tube or passageway, but closes to prevent a reflux or an abnormal flowing back in the opposite direction."

"The plumber turned off the main water valve to the house so he could repair the faucets in the bathroom."

2. Part of a hollow organ; such as, the heart, with a flap to make sure fluid goes through it but doesn't come back: "The most important valves in the body are at the exits from the heart chambers and in the veins."

"By making sure that the blood goes in only one direction, these valves are essential to the circulatory system."

"Without the valves, the heart would not be able to function effectively as a pump and the blood circulation could not take place."

"If the valves in the veins in the legs are not functioning properly, it will cause varicose veins, which are swollen and distorted veins that are on the back of the calf or anywhere on the inside of the legs."

3. A part of brass-wind instruments for changing the lengths of the movements of air in order to change the pitches of musical tones: "When Mark was playing his trumpet, tuba, French horn, or trombone; he was able to play different notes by manipulating the various valves."
4. Etymology: from Latin valva; usually, the plural form, valvae, "a folding door"; related to volvere, "to roll".

The sense of a membranous fold that regulates the flow of body fluids was first recorded in English in 1615, and then the meaning of a mechanical device which works like a valve, was recorded in 1659.

In zoology, the application of one of the halves of a hinged shell was first presented in 1661.

A control valve with a built-in atmospheric vacuum breaker (backflow preventer).

1. An instrument in which electrons move through a vacuum or gaseous medium within a gas-tight chamber; used to generate, to amplify, and to correct electric oscillations and AC currents.
2. An electronic apparatus that consists, typically, of a sealed glass bulb containing two or more electrodes.

It is used to generate, to amplify, and to rectify electric oscillations and alternating currents.

3. An electron device in which electricity is conducted by electrons moving through a vacuum or gaseous medium within an hermetically sealed envelope.

A tube can perform rectification, amplification, modulation, demodulation, oscillation, limiting, and a variety of other activities.

Examples include cathode-ray tubes, gas tubes, phototubes, and vacuum tubes.