The Dungeon Master designs the world the players adventure in, voices the non-player characters and monsters, adjudicates the rules, and steers the story. Other systems use the more general term Game Master (GM). The name "Dungeon Master" is itself a trademark of Wizards of the Coast, so when referring to the general role we pair it with the GM label.
The structure in which one person holds and rules on the world’s laws and outcomes resonates with our narrative form, where the narrator is the order of the world.