A Zener diode is a special type of diode designed to reliably allow current to flow in the reverse direction when a specific voltage, known as the Zener voltage, is reached. This property makes it ideal for use as a voltage regulator. By connecting a Zener diode in reverse bias parallel to a load, it can maintain a constant voltage across the load terminals, even if the input voltage or the load current varies. It achieves this by conducting current in the reverse breakdown region, dissipating excess energy while maintaining a stable voltage.