A Zener diode is a special type of diode that operates obeying the Zener effect. The effect is a type of electrical breakdown in a reverse-biased pn junction — when a sufficiently high reverse voltage is applied, the electric field in the depletion region allows for quantum tunnelling of electrons, which increases reverse current.
We sometimes purposely want devices with this effect, for better control and to operate in reverse breakdown without damage. The Zener breakdown voltage () is generally device-dependent, designed to be within volts. To contrast, non-Zener diodes have a breakdown voltage around volts.