A MUD (Multi-User Dungeon, later also Multi-User Domain or Multi-User Dimension) is a text-based, multiplayer, persistent virtual environment accessed over a network. Players interact with the world and with each other through typed commands: movement, communication, object manipulation, combat, and — in many MUDs — building and programming the world itself.
The first MUD was MUD1 (also called Essex MUD or British Legends), created by Roy Trubshaw and Richard Bartle at the University of Essex in 1978–1980. It ran on a DEC PDP-10 and was accessible over ARPANET. MUD1 established the core features: persistent world state, multiple simultaneous players, text-based rooms connected by exits, objects that could be picked up and used, and NPCs (non-player characters) with scripted behavior.
The MUD family diversified rapidly through the 1980s and 1990s. Major lineages include: AberMUD (1987, Alan Cox), which spread across British universities; TinyMUD (1989, Jim Aspnes), which emphasized social interaction and player-built worlds over combat; LPMud (1989, Lars Pensjö), which introduced the driver/mudlib separation; DikuMUD (1990, University of Copenhagen), which became the most cloned combat-oriented MUD codebase and directly influenced EverQuest and World of Warcraft; and MOO (1990, Stephen White, extended by Pavel Curtis at Xerox PARC), which extended TinyMUD with an object-oriented programming language accessible to all users.
MUDs are historically significant as the first persistent online social spaces, predating the graphical web. They also represent a distinctive model of computing in which the distinction between user, builder, and programmer dissolves — a model that the interactive semioverse formalizes.