What is "triode"

n. A thermionic valve containing an anode, a cathode, and a control grid; small changes to the charge on the grid control the flow from cathode to anode which makes amplification possible

n. a thermionic vacuum tube having three electrodes; fluctuations of the charge on the grid control the flow from cathode to anode which making amplification possible

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 electrode to the thermionic diode ( Fleming valve), the triode was the first practical electronic amplifier 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. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer its sound.

The word "triode" is derived from the Greek τρίοδος, tríodos, from tri- (three) and hodós (road, way), originally meaning the place where three roads meet.