1 : a mechanical device that controls the flow of liquid, gas, etc., by opening and closing
They turned off the main water valve to the house.
2 : a device in some musical instruments (such as trumpets) that you press to play different notes
3 medical : a structure in the heart, stomach, etc., that temporarily stops the flow of fluid or that allows fluid to move in one direction only
a heart valve
4 British : VACUUM TUBE