MUD

Originally known as a Multi-User Dungeon, a MUD is a text-based virtual environment in which users' "characters" interact in real time. Characters can navigate rooms described by text; type to other characters; create shared objects; and engage in games, puzzles, or combat. Most MUDs involve some level of role-playing, where characters adopt personas from books or medieval times, for example. A MUD can be accessed via Telnet or using a special MUD client.