A Zener diode is a type of diode that is designed to operate in the reverse breakdown region. This means that it is designed to allow current to flow in the reverse direction when the applied voltage exceeds a certain threshold, known as the Zener voltage. Zener diodes are commonly used in voltage regulation circuits, where they are used to maintain a constant voltage across a load. They are also used in other applications where stable and predictable reverse breakdown behavior is desired, such as in overvoltage protection circuits. Zener diodes are named after the physicist Clarence Zener, who first described the phenomenon of reverse breakdown in diodes.