Gamemaster

A gamemaster (GM; also known as game master, game manager, game moderator or referee) is a person who acts as an organizer, officiant for questions regarding rules, arbitrator, and moderator for a multiplayer role-playing game. They are most common in co-operative games in which players work together and are less common in competitive games in which players oppose each other. The act performed by a gamemaster is sometimes referred to as "Gamemastering" or simply "GM'ing".