The Oxford Thesaurus An A-Z Dictionary of Synonyms - umpiren.

1) referee, arbiter, judge, moderator, adjudicator, arbitrator; official; Colloq ref, Australian umpy, US ump: The sides agree to abide by the decision of the umpire. --v. 2) referee, arbitrate, judge, moderate, adjudicate; officiate: When Peter Jones umpires a game, the players know they will get a fair decision.