noun
one-shot contest; make-or-break game; single-round match
A contest or situation where everything is decided by a single attempt, round, or decisive moment; no second chances. Often used figuratively for high-stakes, all-or-nothing scenarios.
決勝戦は一発勝負なので、緊張する。
The final is a one-shot contest, so I'm nervous.
この試験は一発勝負だから、しっかり準備しなければ。
This exam is make-or-break, so I have to prepare thoroughly.
Compound of 一発 (one shot, one round) and 勝負 (contest, match). The exact historical derivation is uncertain, but the components transparently combine to mean a contest decided by a single decisive action.