na-adjective
decisive; definitive; conclusive
Describes something that settles a matter, provides final proof, or determines an outcome. Often used with nouns like 証拠, 瞬間, 打撃, 勝利, 違い.
それが決定的な証拠となった。
That became the decisive evidence.
試合は決定的な瞬間を
The match reached a decisive moment.