A thyristor is a semiconductor device that acts as a switch, allowing or blocking electrical current. It has four layers of alternating p-type and n-type materials, forming three junctions. When triggered by a small current at its gate terminal, a thyristor can conduct a large amount of current, making it useful in various applications like motor control and light dimming. Thyristors are commonly used in power electronics due to their ability to handle high voltages and currents. They are often found in devices such as AC power controllers, phase control circuits, and solid-state relays, providing efficient control of electrical power.