A Zener diode is a device used to:

A Zener diode is specifically designed to operate safely and continuously in its reverse-breakdown region. When reverse-biased beyond a specific threshold (its "Zener voltage"), it holds the voltage across its terminals nearly constant, even as the current flowing through it fluctuates. This characteristic makes the Zener diode an ideal component for voltage regulation in power supply circuits, ensuring that delicate downstream components receive a stable reference voltage regardless of input variations. Options suggesting it increases or decreases current, or dissipates voltage, fundamentally misunderstand its purpose; while it does sink variable current to maintain balance, its primary role is to provide a fixed voltage reference.