(MultiUser Dungeon, MultiUser Dimension, MultiUser Dialogue) Interactive games played by several people at a time on the Internet. Originally dungeons and dragon games with demons, elves and magicians, MUDs have been created for science fiction themes, cartoon characters and other types of games. MUDs have also evolved into 3D virtual reality sites.

There are many variations and permutations of MUDs. MOOs are object-oriented MUDs, and MUSEs (Multiuser Shared Environments) are generally designed for elementary and secondary students. A MUSH (MultiUser Shared Hallucination) allows new rooms and situations to be created. A MUCK (MultiUser Chat Kingdom) is a text-based MUD system similar to MUSH, and there is yet another MUCK (MultiUser Construction Kit), heavy on fantasy and myth. See avatar and VRML.