Arbiter

A person appointed, or chosen, by parties to determine a controversy between them.

Any person who has the power of judging and determining, or ordaining, without control; one whose power of deciding and governing is not limited.

To act as arbiter between.

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"