A TRIAC is a three terminal semiconductor. The TRIAC can control the AC power into a circuit by altering its firing angle. The TRIAC differs from an silicon controlled rectifier in that it is bidirectional i.e it can conduct in both directions.
TRIACs can be used in speed control of motors. TRIACs are widely used as fan regulators. TRIACs are also used as light dimmers (See Article on Light Dimmers)