MOOs and MUDs are virtual worlds constructed of words, written text(Lang).
MOO
1) In "Body Language: The Resurrection of the Corpus in Text-Based VR" Candace Lang defines MOO as: "(MOO=MUD Object-Oriented; a MUD is another type of virtual reality game)."
2) According to Jennifer Mnookin in "Virtual(ly) Law: The Emergence of Law in LambdaMOO", she defines MOO as "multi-user, object oriented".
MUD
1) In Objectifying the Body in the Discourse of an Object-Oriented Mud” by Lynn Cherney she defines MUD as a “multi-user dimension”.
2) In Embodying Technesis: Technology Beyond Writing by Mark Hansen asserts that MUD stands for "Multi-User Domains"(16).
3) According to Jennifer Mnookin in "Virtual(ly) Law: The Emergence of Law in LambdaMOO", she defines MUD as "Multi-User Dungeon" or "Multi-User Dimension".