What is zener break down voltage?

The point in the forward operating region of the characteristic curve where conduction starts to increase rapidly is called Knee voltage of a PN Junction Diode.

The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse.(or) Breakdown voltage is a parameter of a diode that defines the largest reverse voltage that can be applied without causing an exponential increase in the current in the diode.

-- Dinakar