A thyristor is a semiconductor device that acts as a switch, allowing or blocking electrical current. It has four layers of semiconductor material and can control high voltages and currents. Thyristors are commonly used in applications like motor control, light dimming, and power regulation. When a small current is applied to its gate terminal, a thyristor can turn on and conduct electricity. Once it is on, it remains in that state until the current flowing through it drops below a certain level. This property makes thyristors useful in various electronic circuits and systems, including AC power control and phase control.