A person who adjudicates.

ETYM Latin arbiter; ar- (for ad) + the root of betere to go; hence properly, one who comes up to look on.
1. Someone chosen to judge and decide a disputed issue; SYN. arbitrator.
2. Someone with the power to settle matters at will; SYN. supreme authority.
3. Judge, especially one chosen by disputing parties. arbiter elegantiae, (Latin) “judge of taste”, especially G. Petronius, supervisor of Nero’s entertainments. + prikaži više

ETYM Latin, from arbitrari: cf. French arbitrateur.
1. A person, or one of two or more persons, chosen by parties who have a controversy, to determine their differences. See Arbitration.
2. One who has the power of deciding or prescribing without control; a ruler; a governor.

(Sports) The chief official (as in boxing or American football) who is expected to ensure fair play; SYN. ref.

ETYM Old Eng. nompere, nounpere (also impier, from French impair uneven), from Old Fren. nomper uneven, French non-pair.
An official at a baseball game; SYN. ump.