Multi-User Dungeon, or MUD (referred to as MUD1, to distinguish it from its successor, MUD2, and the MUD genre in general) is the first MUD and the oldest virtual world in existence.

Multi-User Dungeon, or MUD (referred to as MUD1, to distinguish it from its successor, MUD2, and the MUD genre in general) is the first MUD and the oldest virtual world in existence.