A Multi-User Dungeon (or MUD) is a text-based, multi-player computer game played online. Most MUDs are role-playing games driven by a social chatroom structure. It could be argued that certain online graphics-based games such as World of Warcraft and Eve Online are actually MUDs.
The first MUD was Dungeon, created by Roy Trubshaw and Richard Bartle. This game was inspired by an interactive fiction of the same name, and the goal was to make something akin to a multi-user version of Dungeon, hence the name multi-user dungeon.