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 large amounts of power with a small input signal. Thyristors are commonly used in applications like motor control, light dimming, and power regulation due to their ability to handle high voltages and currents. When a thyristor is triggered by a small current at its gate terminal, it becomes conductive and remains so even after the gate signal is removed. This property makes thyristors useful in circuits where maintaining a steady flow of electricity is essential, such as in AC power control and phase control applications.