三极管是一个有放大器功能的真空管，在内侧真空的玻璃外壳内有三个电极：包括一个加热的灯丝（或称阴极）、控制栅格及金属平板（阳极）。三极管是由李·德富雷斯特在1906年发明，是第一个电子放大器。李·德富雷斯特因为发明三极管，被誉为是电子学之父。

A triode is an electronic amplifying vacuum tube (or valve in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Invented in 1906 by Lee De Forest by adding a grid to the Fleming valve, the triode was the first electronic amplification device and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. The word is derived from the Greek τρίοδος, tríodos, from tri- (three) and hodós (road, way), originally meaning the place where three roads meet.