Crossover (fiction)

A crossover occurs when fictional characters, stories, settings, universes, or mediums meet and interact with each other. For a company, crossovers may exist as a gimmick, a marketing tool, a joke or gag, or to play out a "what if" scenario.

Crossovers of multiple characters from one publisher, especially comicbook publishers, have been used to set an established continuity, where characters can frequently meet within one setting, such as different characters in Marvel comics frequently interacting amongst each other.

Crossovers may also be advertised as a "guest appearance," often to promote another work of fiction, sometimes with little rational explanation.

A crossover story may try to explain its own reason for the crossover, such as "they live next door" or "a dimensional rift brought them together." Some crossovers are not explained at all. Some are absurd or simply impossible within the fictional setting.

Crossover, within continuity

A single instance of one character/group of characters meeting another character/group of characters. These are most likely "in-continuity" to their main respective settings, as the settings do not conflict with each other.

Steve Urkel visits Full House (Full House)
The Critic visits The Simpsons (The Simpsons)
The Jetsons travel back in time to meet The Flintstones (The Flintstones meet the Jetsons)
Go-bots meet The Rock Lords (Gobots: War of the Rock Lords)

Crossover, single-instance within continuity

A crossover exists within one or more main series. The crossover may or may not have taken place, and therefore does not have to be an event that "happened" within either setting. Generally, there is no evidence to support the event ever occured, even if done within a series, and it can be inferred the event existed for its own sake only.

Teename Mutant Ninja Turtles (The Next Mutation version) meets Power Rangers Ninja Force (Power Rangers Ninja Force)
Transformers meet Spiderman (Transformers, Marvel comics)
Conan meets Elric (Conan, Marvel comics)
Ace Ventura meets The Mask (The Mask animated series)
The Mask meets Ace Ventura (Ace Ventura animated series)

Crossover, established within continuity

A crossover occurs multiple times, or is referenced in the main series, implying the crossover actually took place.

Quasar (Marvel comics) travels to the New Universe, and brings the Star Brand back with him
Marvel comics characters meet Ultraverse characters, some Ultraverse characters stay in Marvel universe
Solar (Valiant comics) meets Void of WildC.A.T.S. (Deathmate, Image comics)
Tomb Raider (Top Cow version) meets Witchblade and the Darkness, and assists in reviving Butcher Knight (Top Cow comics)
Batman meets the Darkness (Batman/the Darkness)- note that Jackie Estracado frequently mentions this event, but no explanation is given on how it can occur. It would appear the Top Cow universe has its own version of Gotham City, and not the official DC universe version.
Battle of the Planets (Top Cow version) meets the Thundercats (Top Cow version) (Battle of the Planets/Thundercats, Top Cow)
Multiple SNK characters fight in tournaments together, including characters from Athena, Psycho Soldier, Ikari Warriors, World Heroes, and Fatal Fury (King of Fighters series)

Crossover, self contained outside continuity

These are crossovers that exist of their own setting, as what-if scenarios. Non-continuity crossovers may still borrow or resolve plots from their respective series, but have no evidence to support their occurance officially in either setting.

Archie meets The Punisher
Batman fights Predator (Batman vs Predator I, II, III)
Spawn meets Batman (Spawn/Batman)
Batman meets Spawn (Batman/Spawn)
Robocop fights Terminator
Magnus: Robot Fighter fights Predator
Superman fights Aliens
Superman fights Terminator
Witchblade/The Darkness fight Aliens/Predator
Wolverine meets Witchblade
X-Men meet WildC.A.T.S.
X-Men meet Star Trek: the Original Series characters
X-Men meet Star Trek: the Next Generation characters
Capcom characters fight Marvel characters (X-men vs Street Fighter, Marvel vs Capcom 1 & 2)
Capcom characters fight SNK characters (Capcom vs SNK 1 & 2)

Crossover, individual continuity

A crossover features multiple fictional characters and settings for its own purpose, but there is no evidence of it existing in each respective individual setting.

Multiple Nintendo and third party video game characters exist in N-World, including Megaman, Simon Belmont, Kid Icarus, Eggplant Wizard, Mother Brain, King Hippo, and Link. (Captain N: the Game Master)
Multiple Nintendo and third party video game characters exist solely to fight each other. (Super Smash Bros, Super Smash Bros Melee)

Crossover, dream sequence

A character comes into contact with another character through imagery, dreams, visions, etc. The crossover itself does not occur, but the idea of the crossover might occur.

Spawn sees Cerebus (Spawn)
Gen 13 sees Teenage Mutant Ninja Turtles (Gen 13)

Crossover, celebrity guest appearance

A real life celebrity or figure appears within a fictional setting (Does not have to be portrayed by the actual person).

Multiple celebrities are preserved as talking heads in the 30th century, including Pamela Anderson and Richard Nixon (Futurama)
William Shatner meets Peter Griffin (The Family Guy)

Crossover, gag cameo

A character appears only briefly in another setting, mainly for joke purposes.

Belle from Beauty and the Beast crosses the street in The Hunchback of Notre Dame
Mr. Peabody appears on The Simpsons, commenting on time travel (The Simpsons)
Pumbaa from The Lion King being carried on a spit in The Hunchback of Notre Dame
King of the Hill characters appear on The Simpsons to comment on an amateur baseball game. (The Simpsons)
Rocket J Squirrel appears on The Family Guy, mimicking a familiar scene from Rocky & Bullwinkle (The Family Guy)

Crossover, concepts or ideas

Appearance of items, materials, or objects that appear in another setting. Often used as a non-sequiter or anachronistic gag.

Goofy hat in Aladdin
Aladdin's magic carpet being shaken out in The Hunchback of Notre Dame

Crossover, fan made

Many fan made comics, stories, artwork, and animations feature characters from multiple settings, due to having no impact on official continuity, and for playing out "what-if" scenarios.

Oldskooled, where multiple Nintendo characters hang out at a bar