Zener Breakdown | Zener diode and zener diode as voltage regulator

A Zener diode

What is Zener diode? How is it used as voltage regulator?

Suppose an unregulated D.C. input voltage V_(1) is applied to a Zener diode having breakdown voltage (V_(z)) . Then the breakdown condition for the diode to work as voltage regulator is

Zener diode is used as

The zener diode can be used as