A triode is a type of 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 electrical amplification device and still remains in use for amplification purposes, especially in AF application where the "tried and true" are revered at a premium.
Triodes are now all but obsolete due to transistors but do still find application where reliability and power consumption are not concerns, but low component count and high power capacity are.
Triode
See also