A type of vacuum tube that is used in audio and radio amplifiers and oscillator circuits. It is like a diode with the addition of a wire mesh control grid between the cathode and plate (anode) that controls current flow. A filament heats the cathode enabling it to release electrons. When a small voltage is applied to the grid, the current flow between the cathode and plate is changed accordingly. In some triodes, the filament is the cathode. See diode
The Triode Uses a Grid
Adding a control grid to the diode allows the current to be varied between the cathode and anode.