Arbitrator

A person, or one of two or more persons, chosen by parties who have a controversy, to determine their differences. See Arbitration.

One who has the power of deciding or prescribing without control; a ruler; a governor.

someone chosen to judge and decide a disputed issue; "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"