arbiter

nouns

someone with the power to settle matters at will

"she was the final arbiter on all matters of fashion"

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"

arbiter, arbitrator, umpire

is derived from

arbitrate