1. A device; such as, a klystron, in which the velocity of an electron beam is kept at a constant level in order to produce exceedingly high radio frequencies.
2. Velocity-modulated generator, such as a klystron tube (type of vacuum tube used as an amplifier), used to generate extremely high radio frequencies.

A klystron is an evacuated electron-beam tube in which an initial velocity modulation imparted to electrons in the beam results subsequently in density modulation of the beam. A klystron is used either as an amplifier in the microwave region or as an oscillator.