MUSH (also called MU or MU*) stands for "Multi-User Shared Hallucination,". It is a text-based online game in which players navigate and explore a world of described landscapes and characters. There are other online text-based games called MUD's or MUX's.. but those do not depend on role-play as much as a MUSH does.
No, there are no drugs involved.. the 'hallucination' part just means, its an imaginary world. (An apology to those out there lookin' for a quick fix... unless you're having withdrawl from rp, this won't help you).
Ok ok... If you click on the link below for ELENDOR, it has a wonderful help section that will tell you exactly (even with examples) of what it's all about.. and not just about ELENDOR, but online RP and MU*'s.
ENJOY!!!