MUD

MUD, M.U.D., n., acronym ("mudd") "Multi-User Dungeon" 1. network system software designed to facilitate social interaction between individuals connected over a network or over the Internet; often specifically designed for online RPGs. 2. a game played over a MUD system. Also MUCK, MUSH. [GJ1]
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Addendum: A lot of people argue over what MUD stands for -- Multiple User Dimension, Multiple User Dungeon, Multiple User Dialogue, Many Ugly Dykes -- the list goes on. The original designers of the first MUD called it a Multi-User Dungeon. Get over it. [EDE]
Addendum #2: Growing increasingly rare (though they have made a sort-of comeback in several browser-style games), MUDs are the forerunner to the modern MMO.