Dungeon Master

dungeon master, Dungeon Master, D.M., DM, n., p.n., D&Dism 1. the referee of a Dungeons & Dragons game. 2. generic term for the referee of any fantasy RPG. 3. syn. with, and used interchangeably with, gamemaster or referee. [GJ1]
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Addendum: The term "DM," or "dungeon master," originated with Dungeons & Dragons. Throughout the 80s, "DM" was used as a generic term for RPG referee; but the more generic (and non-copyrighted) "GM," for "gamemaster," slowly replaced it. Today, GM is being replaced with the even more generic "referee," or "storyteller"; see also narrator. [GJ1]