Crossovers occur any time characters from one work "cross over" into another. Any time characters from one work interact with another work, you have a crossover.
This is extremely common in Fan Fic, where the authors take someone else's characters and play with them, often introducing a new setting or new characters in order to better facilitate the story they want to write.
This is an index of the varying types of crossovers; Rule 50 may apply to some of these.
- Crossover (main topic)
- Adaptation Amalgamation (a crossover that's an adaptation of both/all the works being crossed)
- Bat Family Crossover (a crossover that only affects people in one immediate in-story location)
- Canon Welding (an author takes two previously-unrelated works they made and fuses them together into one universe)
- Character Overlap (when certain characters have a tendency to do the crossing over)
- Common Crossover (when two works have a tendency to be crossed over in fanfiction)
- Company Cross References (two works owned by the same company reference each other, or one references another, in-story but they don't actually cross over except for occasions when a character briefly appears)
- Crisis Crossover (a crossover between all the works in a universe)
- Cross Through (similar to a Crisis Crossover but without inter-work character interaction: a story line starts like an installment of one work, then switches to different works within that universe)
- Crossover Alternate Universe (an Alternate Universe that exists solely for the purpose of making a crossover)
- Crossover Combo Villain (when a crossover creates a villain by combining elements from the crossed over works)
- Crossover Cosmology (a crossover between mythologies, e.g. Thor hanging out with Zeus)
- Crossover Couple (two characters from different works fall in love)
- Crossover Finale (the absolute last installment of a work is a crossover)
- Crossover Punchline (a crossover that's just a brief joke)
- Crossover Ship (Like a Crossover Couple but non-canon; shipping two characters from different works together)
- Crossover Villain-in-Chief: When universes collide, one villain rises up to lead the others.
- Deconstruction Crossover (lots of works in one genre are crossed over in order to deconstruct or reconstruct the genre)
- Doppelgänger Crossover (a fanfiction that crosses two works that feature the same actor playing different characters)
- Fairy Tale Free-for-All (a crossover between a lot of fairy tales)
- Fake Crossover (a crossover that is not fanfiction, but not canon either)
- Fully Absorbed Finale (when a show's finale isn't grand because it's wrapped up in an episode of a different show set in the same universe)
- Fusion Fic (a fanfiction where the characters in one work replace the characters in another)
- Guest Fighter (a character from a different work shows up inexplicably in a video game, usually a Fighting Game hence the title)
- Hypothetical Fight Debate (two or more nerds wonder who would win if two characters from different works fought each other)
- Intercontinuity Crossover (a crossover between two works that are not in the same universe)
- Intra-Franchise Crossover (a crossover between adaptations of the same work)
- Lawyer-Friendly Cameo (it's legal to have a crossover without permission if you make it so that technically they're a different character or you censor the character a bit)
- Massive Multiplayer Crossover (a crossover between more than two works)
- Modular Franchise (two universes created by a company become one)
- Monster Mash (popular fictional monsters hanging out with one another)
- Obvious Crossover Method (a work has an obvious method through which crossovers can occur)
- Official Parody (a parody crossover created with the permission of the author of the work being parodied)
- Original Generation (a crossover between lots of works, with a new character as the/a main one)
- Poorly Disguised Pilot (the pilot episode of a spin-off is technically an episode of the show it's being spun off of)
- Power Creep, Power Seep (trouble arises when two works cross and one is too powerful to work with another)
- Red Skies Crossover (similar to a Crisis Crossover, but while all the works in a universe are affected, the characters don't all meet)
- Required Spinoff Crossover (a spin-off and the show it was spun off of do a crossover)
- Sidelined Protagonist Crossover (when crossovers don't involve the main hero of one or more of the involved works or demote them to extra.)
- Sir Cameos-a-Lot: A character who appears in more cameos and crossovers than they do in original works.
- Story-Breaker Team-Up (a crossover that doesn't work very well)
- Spotlight-Stealing Crossover (in a Massive Multiplayer Crossover, one character gets more focus)
- Team-Up Series (a crossover involving heroes teaming up)
- Transplant (a character leaves one work to join another)
- Rogues-Gallery Transplant (a villain introduced in one work crosses over with another and is more associated with that one)
- Ultimate Showdown of Ultimate Destiny (a crossover that answers the Hypothetical Fight Debate)
- Vacation Crossover (a crossover wherein characters meet because one or both parties are on vacation, since they normally live in different places)
- Weird Crossover (two works are crossed that make a bizarre combination)
- The World as Myth (the idea that every work ever created exists in one big fictional multiverse with each fictional universe making up one dimension, possibly even saying that the real world is part of this multiverse)
