Definition - What does Multi-User Dungeon (MUD) mean?
A multi-user dungeon (MUD) is both a style of role-playing game and the name of one of the original text-based multiplayer online role-playing games of this genre. MUD was created by Roy Trubshaw and Richard Bartle in 1980. MUD was modeled after earlier text-based adventure games and became a popular game to play over Telenet, an early commercial Internet.
Multi-user dungeons are sometimes referred to as multi-user dimensions or multi-user domains.
Techopedia explains Multi-User Dungeon (MUD)
As much as any other game, MUD marked the humble beginnings of what would become massively multiplayer online role-playing games (MMORPGs). Multi-user dungeons represented the first virtual worlds on the Internet. When the role-playing elements and dungeon battles used in MUDs went graphical, the popularity of online role-playing spread even further. MMORPGs now represent a mainstream (and profitable) area of gaming.