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decider; person who decides
decisive factor; clincher; key; trump card; winning move; decisive blow; conclusive evidence
今回は決め手に欠ける展開だった。
This time it was a development that lacked a decisive factor.
この試合の決め手は何だったのか。
What was the deciding factor in this game?
転職の決め手は給料ではなく職場の環境だった。
The deciding factor for changing jobs was the workplace environment, not the salary.