Decider (noun)

1. A person who makes a decision or decision-making authority.
2. A person responsible for deciding the outcome of a dispute or competition.

Origin:

From decide, from old french decider, from latin decidere, from de- + caedere ("to cut").

Examples:

1. The chairman was the final decider in all matters of the company.
2. The decider was the referee who made the call on the controversial play.
3. The decider was the judge who decided the outcome of the case.
4. The decider was the jury that rendered the verdict.
5. He was the ultimate decider in the company and had the final say on all important matters.