An arbitrator is the person who officially judges how an argument between two opposing sides should be settled. They listen to both sides and then decide. Not a judge in a public court, though the job feels close. People pick an arbitrator because they know the subject, or because both sides trust them. It is a noun: the person. (The doing is arbitrate; the process is arbitration.) It belongs to work, contracts, and law, not to playground talk. Though actually, there may be one arbitrator or a small panel of three; the word still names each person who has that deciding job, not the meeting itself.