A thyristor is a type of semiconductor device used for switching and controlling electric power. It is a four-layer device that typically has three terminals: the anode, cathode, and gate. Thyristors can conduct current in both directions but are primarily used to control high-voltage and high-current applications, such as in AC power regulation, motor control, light dimming, and over-voltage protection.

Thyristors can remain in a conducting state once triggered, even without a continuous gate signal, until the voltage across the device is reduced below a certain threshold (commonly called the breakover voltage or holding current). This unique feature is the reason they are often used for switching and control applications.