It is a type of thyristor which is designed for electronic control of the supplied current into a circuit, like in dimmers for lighting systems.
A solid state switching device that controls an A.C. load, used in place of a relay in a timer. v Variable external – A timer that is supplied with a timer adjusting potentiometer separate from the main unit. Two terminals for connecting wires are on the timer.
A gated switching device which will conduct in either direction.