The thyristor family
Diac: Diac (diode for alternating current) is a component which starts conducting at a certain voltage (Vbo). It is used in power electronics and is often combined with triacs.
Triac: Triacs (triode for alternating current) are used in AC applications, for instance for regulating power in a heating element, motor, lamp etc. This is achieved by controlling the trigger input or gate, of the triac. When the input is on (positive or negative) the triac opens, and closes when the voltage on the gate returns to zero.
Thyristor: The thyristor is similar to the triac, but only conducts current in one direction.