A person who intervenes in a dispute to help the parties reach a resolution, often in legal, business, or interpersonal contexts.
He was chosen as the arbitrator for the labor dispute.
The mediator helped the two parties talk things over.
Compound of 仲裁 (arbitration, mediation) and 者 (person). 仲裁 is from 仲 (relationship) and 裁 (judge).