In electronics, a vacuum tube, electron tube, or (outside North America) thermionic valve or just valve, is a device used to amplify, switch or modify a signal by controlling the movement of electrons in an evacuated space. Vacuum tubes were critical devices in electronics technology, leading to the development and commercialization of such technologies as radio broadcasting, television, radar, high fidelity sound reproduction, telephone, analog and digital computers, and industrial process control.
For most purposes, the vacuum tube has been replaced by the much smaller, more efficient, and less expensive transistor, either as a discrete device or in an integrated circuit. However, tubes are still used in specialized applications, such as in high-end audio systems, musical instrument amplifiers and high power radio frequency transmitters. Cathode ray tubes are still used as display devices in television sets and video monitors, although they are slowly being replaced by LCDs and other flat-panel displays. A specialized form of the electron tube, the magnetron, is the source of microwave energy in microwave ovens and some radar systems.