In general usage, mediate and arbitrate are synonyms. However, as a reader pointed out when I used the words mediator and arbitrated in a sentence illustrating the difference between uninterested and disinterested, the roles of mediator and arbitrator in a legal context are distinct.
Note: Like arbitrator, the noun arbiter also means “one who judges.” Arbiter usually refers to someone who judges matters of taste or etiquette: “Gradually, the arbiters of the New York art world caught on. Superlatives in The New York Times became almost routine.”
The reader, who has served as both mediator and arbitrator, explains the difference this way:
As a mediator, I help to facilitate a resolution of matters in dispute, a resolution…that all parties then agree to. I do not decide the matter, the parties do.
As an arbitrator, I act as a judge – although I consider the parties’ respective proposals for resolution, I decide how the matters will be resolved.
Because litigation is time-consuming and extremely expensive, processes called mediation and arbitration have become popular as alternatives or adjuncts to litigation.
litigation: any lawsuit or other resort to the courts to determine a legal question or matter.
mediation: an attempt to settle a legal dispute with the help of a mediator (neutral third party) who works with the disputants to find points of agreement and reach a fair solution.
arbitration: an informal trial presided over by a person or panel of persons (neutral third parties) who are not judges in the judicial system.
Mediation may or may not result in a satisfactory settlement. With arbitration, the disputing parties (usually) agree in advance to accept the decision of the arbitrator/s.
Sources: FindLaw.com and Law.com