Dungeon, Domain, Dimension have all been used in the past for the MUD acronym.

Bartle, Richard (2003). Designing Virtual Worlds. New Riders. pp. 9–10, 741. ISBN 0-13-101816-7. [pp. 9-10] TinyMUD was deliberately intended to be distanced from the prevailing hack-and-slay AberMUD style, and the “D” in its name was said…