MUD (acronym) - Multi-User Dungeon
Before the days of MMORPGs a form of text-based gaming arose from the popularity of games on mainframes and early PCs. These games, such as Infocom's Zork, featured dungeon-crawling, puzzle-solving and, sometimes, combat. Navigation was accomplished by a command and response mechanism where the player told the game what he wanted to do and the game responded with a description of the result and a list of possible options/commands to proceed.
Many Bulleting Board Systems (BBSs) featured the first online adaptations of these games, which were known as MUDs.