decider

noun

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.”

   “The Determination of his Majesty, who is the only proper decider of this Matter.”

   “The god Adar, which, with its two oft-occurring idiographs Bar and Nin-ib, is preferably designated as the "Decider" (Entschneider).”

   “Although the decider may know any of the principles in the sequence, he cannot know every such principle.”

   “I have strong confidence in Don Rumsfeld. I hear the voices. And I read the front page. And I know the speculation. But I'm the decider and I decide what is best.”

2. (Australia, British)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.”

   “[…] when the Welshman laid on the 74th-minute decider.”

   “Tensions threatened to boil over before the defending champion Gerwyn Price eventually overcame Kim Huybrechts in a sudden-death decider to reach the last 16 of the PDC World Championship.”

3. A Turing machine that halts regardless of its input.