A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube (or valve in British English) with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device, which propelled the electronics age forward, by enabling amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics until the 1950s, when bipolar junction transistors replaced them. The word is derived from the Greek τρίοδος, tríodos, from tri- (three) and hodós (road, way), originally meaning the place where three roads meet.

The original three-element device was patented in 1907 by Lee De Forest who developed it from his original two-element 1906 Audion. The Audion did provide amplification. However it was not until around 1912 that other researchers, while attempting to improve the service life of the Audion, stumbled on the principle of the true vacuum tube. The name triode appeared later, when it became necessary to distinguish it from other generic kinds of vacuum tubes with more or fewer elements (e.g. diodes, tetrodes, pentodes etc.). The Audion tubes deliberately contained some gas at low pressure. The name triode is only applied to vacuum tubes which have been evacuated of as much gas as possible.