a sealed tube, usually of glass, containing a vacuum or a gas and electrodes that are connected to pinlike terminals that protrude from the base, used as a plug-in component in an electronic device to control the flow of electrons
See electron tube in American Heritage Dictionary 4
noun
A sealed enclosure, either highly evacuated or containing a controlled quantity of gas, in which electrons can be made sufficiently mobile to act as the principal carriers of current between at least one pair of electrodes.