The Free On-line Dictionary of Computing (30 December 2018):

Multi-User Shared Hallucination

    (MUSH) A user-extendable MUD.
   A MUSH provides commands which the players can use to
   construct new rooms or make objects and puzzles for other
   players to explore.

   (http://cis.upenn.edu/~lwl/muds.html).

   (1995-03-16)