MUD

From StarWiki

A MUD, or multi-user dungeon, dimension, or sometimes even domain is a multi-player computer role playing game. Players assume the role of a character, and see textual descriptions of rooms, objects, other characters, and computer-controlled creatures or non-player characters (NPCs) in a virtual environment. They may interact with each other and their virtual surroundings by typing commands, usually in an abbreviated form of English.