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The noun pivot is a person, thing, or factor having a major or central role, function, or effect (der Drehpunkt, der Drehzapfen, der Schwenkungspunkt).

So the adjective is used to describe something which is vitally important.

NOTE:

As it's important to discuss important things, you might like to note this bunch of often-used synonyms :

cardinal, central, chief, critical, crucial, decisive, determining, essential, focal, foremost, fundamental, greatest, highest, key, leading, main, middle, momentous, overriding, paramount, PIVOTAL, pre-eminent, primary, prime, principal, ruling, vital