Crossover

A fictional crossover is a type of link between stories.

It happens when a story is created purposely to merge two or more otherwise discrete fictional stories in one. An example is Freddy vs Jason, a movie created to merge the stories of Nightmare on Elm Street and Friday the 13th.

It might happen that the crossover reveals that the involved stories always happened in the same universe (such as in Freddy vs Jason) or that it is an extra-dimentional story that involves different universes (such as in Mortal Kombat vs. DC Universe). Sometimes, even if the story is supposed to merge two universes, it is ignored in the main canon of the stories (Still Mortal Kombat vs. DC Universe is an example), thus actually taking place in two (or more) new universes.