What is "instance dungeon"

n. (context online gaming English) In massively multiplayer online games an instance of a given dungeon generated exclusively for a specific player or party of players; contrast to the overworld, shared by all players on the server.

In massively multiplayer online games, an instance is a special area, typically a dungeon, that generates a new copy of the location for each group, or for certain number of players, that enters the area. Instancing, the general term for the use of this technique, addresses several problems encountered by players in the shared spaces of virtual worlds. It is not widely known when instances were first used in this genre, however The Realm Online (1996) is sometimes credited as introducing the concept.