occur any time characters from one work "cross over" into another. Any time characters from one work interact with another work, you have a crossover.
This is extremely common in Fan Fic
, where the authors take someone else's characters and play with them, often introducing a new setting or new characters in order to better facilitate the story they want to write.
This is an index of the varying types of crossovers; Rule 50
may apply to some of these.