MUD

Wikipedia as at 21st April, 2011, describes a MUD as
"originally a Multi-User Dungeon, a MUD is a multiplayer real-time virtual world described primarily in text, MUDs combine elements of roleplaying games, hack and slash, player versus player, interactive fiction, and online chat. Players can read or view descriptions of rooms, objects, other players, non-player characters, and actions performed in the virtual world. Players typically interact with each other and the world by typing commands that resemble a natural language."