reverse bias

Reverse bias is connecting the diode the 'wrong' way — positive to the N side (cathode), negative to the P side (anode). Now the diode refuses to conduct; it behaves like an open switch. This is the blocking half of the one-way valve, and it is exactly what makes a diode useful for steering and rectifying.

Mechanism: the external voltage pulls holes and electrons AWAY from the junction, so the depletion region grows wider and the built-in barrier grows taller. Almost nothing crosses. There is a tiny reverse leakage current — nanoamps to microamps, rising with temperature — but for most purposes it is zero. The diode is now a small capacitor (two charged regions separated by the insulating depletion layer) more than a conductor.

But blocking has a limit. Crank the reverse voltage high enough and the junction breaks down and conducts hard in reverse, at the 'peak inverse voltage' or breakdown voltage. For an ordinary diode that is a failure to avoid (pick a part rated well above your worst-case reverse voltage); for a Zener diode, breakdown is the whole point and is used on purpose.