Crossover

A crossover is "the placement of two or more otherwise discrete fictional characters, settings, works, or universes into the context of a single story." Official crossovers come from shared corporate ownership or agreements between rights holders, an "intercompany crossover" when rival companies strike the deal; unofficial ones are fan-made, and the device differs from parody, where one universe merely copies another. Early literary examples include Tom Sawyer walking into The Adventures of Huckleberry Finn and Verne crossing two of his own novels in The Mysterious Island.

For the working writer a crossover is a rules collision: every imported setting carries its own canon and power scale, and the story must reconcile them or the seams show. Fan fiction communities treat the unofficial crossover as a standard, rights-free form, and long serials stage internal versions of the same move whenever separate casts or storylines finally meet.