Glossary:Crossover

Crossovers of multiple characters have been used to set an established continuity, where characters can frequently meet within one setting. This is especially true of in the Marvel Universe, as different characters frequently interact with one another since they live in the same "universe". For example, the X-Men have frequent dealings with another group of Marvel heroes, such as the Fantastic Four. In comic book terminology, these "guest star" roles are common enough that they are not considered crossovers. A crossover in comic book terms only occurs when a story spans more than one title. This has led to "crossover events", in which major occurrences are shown as affecting (almost) all the stories in the shared universe.