"MUD (Multi-User Dungeon or Dimension)"

Definition: MUD (Multi-User Dungeon or Dimension): A (usually text-based) multi-user simulation environment. Some are purely for fun and flirting, others are used for serious software development, or education purposes and all thatlies in between. A significant feature of most MUDs is that users can create things that stay after they leave and which other users can interact within their absence, thus allowing a world to be built gradually and collectively. From Matisse

* Linux/Unix/Computing Glossary