dispositive

Dispositive describes something that settles a matter conclusively or decisively.

In legal contexts, a dispositive fact or issue is one that determines the outcome of a case: The judge ruled that the unsigned contract was dispositive.

Outside law, dispositive retains this sense of being decisive or determinative: Her testimony was dispositive—after that, no one doubted what had happened.

The word derives from dispose in its sense of settling or arranging matters finally.