MUD is an acronym for multi-user dungeon, or in later years multi-user dimension and multi-user domain. A MUD is a real-time text-based virtual world, usually in the form of some kind of role playing game.

Genres range from scifi and fantasy to anime and real-life simulations.

In Sweden, the most notable ones are SvenskMUD and Gryningstid, while internationally, ZombieMUD and NannyMUD gained a lot of popularity.

While MUDs still exist today, it is not really attracting that many new players. Most players still active are there for nostalgic reasons.