MUD 
Multi-User Dimension 
A virtual environment, either text based or involving 3-D graphics, in which users adopt a character (also called an avatar) and interact in real time. Most MUD games involve some level of role playing, where characters adopt personas from books or medieval times, for example. A MUD can be accessed via Telnet, with a MUD client, or via a Web site. The acronym originally stood for Multi-User Dungeon.