metaplot, n., an over-arching plot in a given role-playing game, written into its supplements and modules, that generally evolves over time.
Addendum: Metaplots are often hard to follow in some RPGs, since you need most of the supplements and other books to keep up with it sometimes. While there's no requirement in any game to actually follow this plotline, many GMs and players obsess over it, (see Encyclopaedia WoDica and story lawyer) mostly as a way to show off their knowledge.
Addendum #2: However, many players enjoy the evolving metaplot in a game, as both motivation to buy more books, and a feeling that the game is continually being refreshed.