Multi-User Dungeon or Dimension
a.k.a. "Multi-User Dimension"
A multi-user simulation environment, usually text-based. It incorporates an object-oriented programming language that participants use to construct their own characters and worlds. Some worlds are purely for fun and flirting, but others are used for serious software development, educational purposes, and all that lies in between. A significant feature of most MUDs is that users can create things that stay after they leave and that can be further developed in their absence, thus allowing a world to be built gradually and collectively.
NetLingo Classification: Net Technology