ID:178573
 
It stands for Multi User Dungeon. It's an old school term for massively multiplayer online games. Usually MUDs are thought of as the text variety of the online games, but sometimes the name is used as a cover-all for all online games.
In response to Likwiddraino000
Though I prefer the definition 'Multiple User Dimension,' because many MUDs have nothing or little to do with dungeons.

-LoW
In response to Lord of Water
I prefer MPU...

Multiple Player Universe

Because a lot of MUDs don't take place on only one planet.

for examples of muds check out http://www.mudconnect.com/