MerriamWebster: a contingent event or condition: such as an event (such as an emergency) that may but is not certain to occur trying to provide for every contingency; something liable to happen as an adjunct to or result of something else, the contingencies of war.

A contingency plan, also known colloquially as Plan B, is a plan devised for an outcome other than in the usual (expected) plan. It is often used for risk management for an exceptional risk that, though unlikely, would have catastrophic consequences.