A person or thing making a decision could be described as being a decider or a decision maker. More commonly we would use decision maker to describe a person making a decision. Whos idea was this? John was the decision maker! Decider can also be used when we are talking about the outcome of an event, particularly sporting events or quizzes, that sort of thing. It is used when the final outcome is still to be decided, but the outcome rests on a final event. For example: They have played two matches so far. John has won one, Fred has was one. The last match will be the decider (i.e. the final match will decide the overal winner) Will John win the money? This final question will be the decider! Answer correctly he will win, answer incorrectly he will get nothing.