A TRIAC, or triode for alternating current, is a three terminal electronic switch that conducts current in two directions. Because they can switch current in both directions, they are useful for switching alternating current and controlling the average current that flows into a load. This allows triac control components to control the speed of motors, dim lamps, and adjust electric heaters.
Triacs are similar to thyristors in that they are used as solid state switching devices, but with the added benefit of being able to trigger with both positive and negative voltages applied to the anode. This allows for full-wave power control without additional components or complex wiring.
Bidirectional switching makes triac controls ideal for basic AC switching to start/stop a motor or turn on/off a lamp. They are also capable of phase control and can vary the amount of power supplied to a load, useful for motor speed control applications.