MUSH - a Multi-User Shared Hallucination. A flavor of multi-user text-based network game. Variants include TinyMUX, PennMUSH, and TinyMUSH. They all used variants of a programming language called MUSHCode.

Since MUSH does not have a built-in combat resolution system, games usually emphasize cooperative roleplaying (which is very hard) and player interaction over fighting. Some games have implemented custom combat systems in the aforementioned MUSHCode, but these will vary wildly from game to game.