A zener diode is a type of diode that is designed to operate in the reverse breakdown region of its voltage-current characteristic curve. In simpler terms, it is a diode that allows current to flow in the reverse direction when a certain voltage threshold is reached. This characteristic makes it useful for voltage regulation applications. When used in a circuit, a zener diode can maintain a nearly constant voltage across its terminals, even if there are changes in the input voltage or load resistance. Therefore, the correct option is "voltage regulation".