A mediator is a person who helps negotiate between two feuding parties. When a married couple is considering getting divorced, they sometimes hire a mediator to help them come to an agreement, and possibly even avoid divorce.
The word mediator goes back to the Latin word medius, which means middle. A mediator is supposed to remain in the neutral middle rather than taking one side over another, in order to help both sides resolve a dispute. Mediators negotiate between employers and employees, disputing couples, and in schools helping kids work out their conflicts peacefully rather than fighting on the playground or lunchroom.