A MUD (; originally Multi-User Dungeon, with later variants Multi-User Dimension and Multi-User Domain), is a multiplayer real-time virtual world, usually text-based. MUDs combine elements of role-playing games, hack and slash, player versus player, interactive fiction, and online chat. Players can read or view descriptions of rooms, objects, other players, non-player characters, and actions performed in the virtual world. Players typically interact with each other and the world by typing commands that resemble a natural language. Traditional MUDs implement a role-playing video game set in a fantasy world populated by fictional races and monsters, with players choosing classes in order to gain specific skills or powers. The objective of this sort of game is to slay monsters, explore a fantasy world, complete quests, go on adventures, create a story by roleplaying, and advance the created character. Many MUDs were fashioned around the dice-rolling rules of the Dungeons & Dragons series of games.

Mud is a mixture of water and any combination of soil, silt, and clay, and usually forms after rainfall or near water sources. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone (generally called lutites). When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds.