Equipment grounding conductor

OSHA defines it as the conductor that connects the noncurrent-carrying metal parts of equipment, raceways and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment. Put simply, it gives a fault a path back to the panel so the breaker trips.

This wire is why the third prong on a three-prong outlet matters. It works with bonding. Older homes may lack it, as with an ungrounded outlet. A licensed electrician should assess and correct this.