run-off election; final ballot
Refers to a second, decisive round of voting when no candidate achieves a required majority in the first round. Common in political contexts.
大統領選挙は決選投票にもつれ込んだ。
The presidential election went to a run-off vote.
決選で彼が当選した。
He was elected in the run-off.
決選 (also read けっせん) refers to a runoff vote or ballot, while 決戦 means a decisive battle or match; they sound identical but mean very different things.
final vote; run-off ballot
Compound of 決 (decide) and 選 (choose/elect), literally 'decisive selection'. The reading けっせん follows standard on'yomi.