A Multi-User Dungeon (usually MUD) is a text adventure game where multiple users can interact with each other.
Other influential implementations were AberMUD and LPmud. The former was written in B for a Honeywell mainframe at the University of Aberystwyth. Programmers include Alan Cox of later Linux fame. LPmud was written in C by Lars Pensjö at Chalmers Technical Institute. It features a built-in C-like object-oriented extension language, LPC.