Acronym for Multi-User Dungeon. A MUD is a MMORPG(also an acronym for Massive-Multiplayer Online Role Playing game) that is mainly text-based. Most MUDs are coded in C++ and free to both play and use. Players can connect to a MUD server via Telnet, ZMUD, MUSHclient, or GMUD.
"One of the more successful MUDs today would be Achaea."