Multi-user dungeon, basically a text-based MMORPG.

https://en.wikipedia.org/wiki/Multi-user_dungeon

A text-based, multiplayer, real-time virtual world. Sometimes more group-chat, sometimes more adventure story scenario: https://en.wikipedia.org/wiki/Multi-user_dungeon

MultiUserDomain - a precursor to today's MMO. You could connect to an ascii based adventure/roleplaying game that others could connect as well and communicate. I played astaria.net back in the late 90s.

Multi-User Dungeon; Cf. MUCK for multi-user chat kingdom and friends. Essentially a shared text-only set of chat rooms with a theme and some built-in game logic.

Multi User Dungeons were sort of multiplayer roguelikes/interactive fiction. The text-based precursors to MMOs