TRIode for Alternating Current. A thyristor used in phase control dimming to switch the AC waveform to vary the amount of power to a load. When current is applied to the gate (G) it switches on allowing current to be conducted bidirectionally between A1 and A2. Once switched on, the triac will remain on even when there is no current being applied to the gate. However once the current between A1 and A2 drops below a certain value called the holding current, such as at the end of an AC half cycle, the triac switches off.
Commonly found in inexpensive stage dimmers, as well as the ones found controlling the chandelier in dining rooms across America.