n. A person, divinity, or authoritative text which decides.
n. An event or action which decides the outcome of a contested matter.
n. A Turing machine that halts regardless of its input.
the GNU version of the Collaborative International Dictionary of English
n. One who decides.
The Century Dictionary and Cyclopedia
n. One who decides; one who or that which determines a cause or contest.
Word Usage
"United will now look ahead to Wednesday's Champions League semi-final second leg ahead of what could prove to be a title decider against Carlo Ancelotti's Chelsea."