A person or thing that makes a final decision or settles a matter; in sports, a contest or goal that determines the winner.
From the verb 'decide' plus the suffix '-er' (one who does the action). It's a modern, straightforward formation in English.
In sports, a 'decider' is dramatic—it's the final game or the sudden-death goal that ends everything, like the seventh game of a World Series deciding which team takes it all!
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