MUD

Short for Multi-User Dungeon (or Multi-User Dimension) a cyberspace where users can take on an identity in the form of an avatar and interact with one another. Originally, MUDs tended to be adventure games played within enormous old castles with hidden rooms, trap-doors, exotic beasts, and magical items. Nowadays, the term is used more generically to refer to any cyberspace. MUDs are also known as 3-D worlds and chat worlds.