arbiter

(noun) someone with the power to settle matters at will. Synonyms: supreme authority. Example: "She was the final arbiter on all matters of fashion."

(noun) someone chosen to judge and decide a disputed issue. Synonyms: arbitrator, umpire. Examples: "The critic was considered to be an arbiter of modern literature." "The arbitrator's authority derived from the consent of the disputants." "An umpire was appointed to settle the tax case."