decider

noun3

1. •
   A person, divinity, or authoritative text which decides.

   • This written and revealed will of God I said was the Judge and Decider of all Questions. — 1667
2. •
   (UK, Australia) An event or action which decides the outcome of a contested matter.

   • Four years on, France […] will meet Ireland again in the probable decider for their World Cup pool in September 21 in Paris. — 2007
3. •
   A Turing machine that halts regardless of its input.