Zener diode
A Zener diode is a special diode designed to be run backwards on purpose. An ordinary diode breaks if you reverse-bias it hard enough; a Zener is built so that at a precise reverse voltage — say 5.1 V or 12 V — it suddenly conducts in a controlled, non-destructive way, clamping the voltage across it like a wall a ball bounces off. It is the cheapest way to pin a node to a known voltage.
Because that breakdown voltage is sharp and stable, a Zener plus one resistor makes a simple voltage reference or a crude regulator: any extra current the load doesn't take, the Zener swallows, holding the output near its rated voltage. Below ~5 V the mechanism is true Zener (quantum tunnelling); above ~6 V it is actually avalanche breakdown — but both types are sold under the 'Zener' name.
Around 5–6 V the two mechanisms cancel each other's temperature drift, so ~5.6 V Zeners are the most temperature-stable and were long used as voltage references before bandgap references took over.