Crossover (or X-over)

Stories in which the characters, premises, or settings of more than one fandom coincide. Crossovers may consist of a complete blending of universes or only a slight, passing connection. Knowledge of both (or multiple) fandoms is not always necessary, but generally helpful to fully understanding the story.

I read a crossover story where Sebastian and Ciel were just casually in the Death note universe, and they were dating. Like Omg I freaked!