A player that runs most aspects of a tabletop roleplaying game. As narrator they describe the world and set the scene. They act as arbiter of the rules by adjudicating player actions and making the final call on rule disputes. Because the DM simulates the entire game-world they control almost all non-player characters and enemies. It’s the DM’s job to provide the adventuring party with quests and obstacles to interact with. When the party comes across trouble the DM runs the combat encounter and decides what actions the enemy characters take.
aka DM, Game Master, GM.