A crossover is a story where two or more discrete franchises, characters or universes are put into the same work. One of the rules of the Internet, Rule 50, states that “A crossover, even improbable ones, will eventually happen in fan art, fan fiction, or official content. NO EXCEPTIONS.”
The concept of the crossover has been present in media for a long while, one of the earliest known examples being a story entitled “Sherlock Holmes Arrives Too Late”, which pitted Sir Arthur Conan Doyle’s famous detective against Maurice Leblanc’s character Arsène Lupin, the gentleman thief. This was an unofficial crossover, originating in the french magazine ‘Je Sai Tout’ in Juine 1906. When Doyle found out about the use of his character, he requested that the name in the story be changed to Herlock Sholmes in order to avoid a legal situation.
Ever since then, crossovers have been prominent in every section of media, from literature to movies to television to even video games, in both official and unofficial works.
The concept of crossovers often draws heavy criticism, with some viewing the practice as a cop-out or a way to cash in on the success of two different series. However, others view it as a creative opportunity to see how different franchises work off of one another, in terms of both character and story.
Whatever the case, people just seem to love the idea of mixing together different things too see what happens. And with all of the different possibilities out there, it’s pretty safe to say that the practice will never die out.
Related practices include Mega-Crossovers, in which several different franchises are used, Crossover Shipping, which puts together two characters from different universes, and Crisis Crossovers, which are pretty much exactly what they sound like.