In sport, a decider is the last game, match, or part of a competition that shows who the winner is. Two sides may have won the same number of games; the decider is the extra one that breaks the tie. A best-of-three series can end in a decider. This use is especially at home in British and Commonwealth English. American English often says the deciding game instead. This entry is not a person. Someone who chooses is a decision-maker, or just a person who decides. Calling that person “a decider” will sound odd in class, even though you can see how the pieces would suggest it. The word is informal-to-neutral sports talk, not a formal legal word.