Fictional crossover

A fictional crossover (short: crossover) is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story. They can arise from legal agreements between the relevant Copyright holders, unauthorized efforts by fans, or common corporate ownership.

Official crossovers
Crossovers often occur in an official capacity in order for the intellectual property rights holders to reap the financial reward of combining two or more popular, established properties. In other cases, the crossover can serve to introduce a new concept derivative of an older one.

Crossovers generally occur between properties owned by a single holder, but they can, more rarely, involve properties from different holders, provided that the inherent legal obstacles can be overcome. They may also involve using characters that have passed into the public domain with those concurrently under copyright protection.

A crossover story may try to explain its own reason for the crossover, such as characters being neighbors (notable examples being the casts from Golden Girls and Empty Nest) or meeting via dimensional rift or similar phenomenon (a common explanation for science fiction properties that have different owners). Some crossovers are not explained at all. Others are absurd or simply impossible within the fictional setting, and have to be ignored by the series' respective continuities. Still others intentionally make the relations between two or more fictional universes confusing, as with The Simpsons and Futurama, where each show is fiction in the other.

Comics
Crossovers of multiple characters owned by one company or published by one publisher, have been used to set an established continuity, where characters can frequently meet within one setting. This is especially true of comic book publishers, as different characters in various Marvel, DC or Valiant comic books frequently interact with one another since they live in a "shared universe". For example, in the Marvel Comics universe, Spider-Man has frequent dealings with another Marvel hero, Daredevil, just as in the DC Comics Universe, Batman and Superman often collaborate. In comic book terminology, these "guest star" roles are common enough that they are not considered crossovers. A crossover in comic book terms only occurs when a story spans more than one title. This has led to "crossover events", in which major occurrences are shown as affecting most or all of the stories in the shared universe.

The earliest such crossover event was Gardner Fox's Zatanna's Search, which took place in Hawkman #4 (October/November 1964), Detective Comics #336 (February 1965), The Atom #19 (June/July 1965), Green Lantern #42 (January 1966), Detective Comics #355 (September 1966), and Justice League of America #51 (February 1967). This story dealt with Zatanna attempting to reconnect with her father, Zatara, and seeking the aid of Hawkman, Batman, Robin, The Atom, Green Lantern, and Elongated Man along the way.

The first major crossover event was spearheaded by the Marvel Editor-in-Chief at the time, Jim Shooter. As a way to further toy sales he devised the Secret Wars crossover which brought all the major Marvel heroes into a twelve issue mini-series to battle a common threat. After the threat was dealt with they all returned to their regular titles. This Secret Wars was hailed as both a critical and commercial success largely because the events of the crossover had lasting effects on the characters (such as the introduction of Spider-Man's black suit which would later become the villain Venom). Jim Shooter later perfected his crossover at Valiant Comics with the Unity event. Unity brought all the Valiant characters together to defeat Mothergod but was told within the existing Valiant Comics titles (and two bookend special issues). Readers were not obliged to buy all 18 chapters as the story was coherent when reading just one title, but far more layered when all were read. Like Secret Wars, the Unity crossover had lasting effects on the Valiant universe most notably the introduction of Turok, the birth of Magnus Robot Fighter and the death of a major Valiant hero.

Dark Horse Comics's Aliens Versus Predator franchise was a success that continued into many games and two movies and even an Aliens Versus Predator Versus The Terminator comic.

The comic crossovers from Raj Comics are very famous in India, in which the super heroes meet to fight a common enemy. Many of these crossovers have occurred between Nagraj and Super Commando Dhruva. In Kohram, all the heroes in Raj Universe meet to finish Haru, an extremely powerful enemy.

Webcomics creators sometimes produce crossovers; one of the first was a two-week sequence between Christopher Baldwin's Bruno and Peter Zale's Helen in 1998.

In 2013, Archie Comics released a 12-part crossover of Capcom's character Mega Man, and Sega's character Sonic the Hedgehog called "Worlds Collide".