The generic term for any of the multiple-user environments that tend to be named with acronyms beginning with MU. They are derived from MUD, or Multiple User Dimension, and now exist in variants such as MUSH, MUX, MUCK, MOO, etc.

The term MUD is also used generically in the same way. However, this sometimes creates confusion because many people consider MUD to refer only to the servers such as CircleMUD, DikuMUD, or LPMUD, where the code is mostly based on Dungeons and Dragons rules - the kind that EverQuest stuck the pretty graphics on.

One would think MU* would be more general because of the wildcard, but in fact it tends to only refer to servers that are derived from TinyMUD.

MUD history is insanely convoluted, and this description doesn't do it justice. If you search around, you can find sites that attempt to diagram the evolution of the modern MU* codebases - even the diagrams are confusing.