1. A large, usually abrupt rise in electric current in the presence of a small increase in voltage; can occur in a confined gas between two electrodes, a gas tube, the atmosphere; such as, lightning, an electrical insulator, and a reverse-biased semiconductor diode.
2. A large, usually abrupt, rise in electric current in the presence of a small increase in electric voltage.

Breakdown may be intentional and controlled or it may be accidental; for example, lightning is the most familiar example of a breakdown.