Crossover

A crossover is a fanfiction that combines two or more fandoms. They range from the almost likely to the completely impossible.

Types of Crossovers
Actor Crossovers An Actor Crossover typically involves a character meeting another character played by the same actor. This type often results from the same actor playing parts on two popular television shows or movies. (For example, David Tennant in Doctor Who and Harry Potter and the Goblet of Fire, or Derek Jacobi in The Golden Compass and I, Claudius.) There are usually many bad jokes about how the two characters look like each other. Another type of Actor Crossover is a variety of Real Person Fic in which an actor enters a canon continuum and meets the character he plays. This type is less common, but also involves bad jokes.

Blender Crossovers According to Architeuthis, "different canonical characters from different continuums are "revealed" to be actually the same person". "Well, Gandalf and Dumbledore are both magic users with long beards, so ... wow, they must be the same person!"

Canon Crossovers This rare type is a crossover that takes place in the actual canon. It usually occurs when two or more television shows are owned by the same network or when a show has several spinoffs. The Disney Channel is rife with canon crossovers.

Standard Crossovers The most common kind, a standard crossover occurs when a fanfic author smashes together two continua that are not canonically connected. This can be done very successfully, or it can leave the reader going "WTF?" The Harry Potter continuum has very many standard crossovers. It is a generally acknowledged rule that if a fandom exists, it has been crossed with Harry Potter.

Crossovers of the WTF? variety are dealt with by the Department of Implausible Crossovers (DoIC), which is headed by the Lichen.