An electron vacuum tube (UK), or tube (North America), is a device that conducts an electric current in a large vacuum between electrodes across which an electric potential difference is applied. This type, known as a thermionic tube or thermionic valve, uses thermionic emission of electrons from a hot cathode for basic electronic functions such as signal amplification and current rectification.

The simplest diode vacuum tube, invented in 1904 by John Ambrose Fleming, contains only a heated electron-emitting cathode and an anode. In the 1940s, the invention of semiconductor devices made it possible to produce solid-state devices that are smaller, more efficient, more reliable, more durable, safer, and more affordable than thermionic tubes. From the mid-1960s, thermionic tubes were replaced by transistors.

However, the cathode ray tube (CRT) remained the basis of television monitors and oscilloscopes until the early 21st century. Thermionic tubes are still used in some applications, such as magnetrons used in microwave ovens, some high frequency amplifiers, and amplifiers preferred by audiophiles for their “warmer” sound.