A crossover occurs when characters published by different companies or perceived as belonging to different fictional realities appear in a shared context. Such crossovers expand an original world into a shared universe. So many crossovers now exist that pretty much any fictional character can be connected to any other, often with far fewer than the six degrees of separation.
I have no idea who first noted The White Shadow/The X-Files crossover (I have read it from one Allan W. Fix online and heard it from a Sarnian librarian). It begins with Byron Stewart, who appeared in the 80s teen show, The White Shadow. Through a variety of in-jokes and script-monkeying, his recurring character on St. Elsewhere became the same character as the one on The White Shadow.
St. Elsewhere was expressly crossed-over with Homicide: Life on the Street when Elfre Woodard's character, Dr. Roxanne Turner appeared. Although The X-Files has been referred to as a fictional show on Homicide, its Detective John Munch (Richard Belzer) has appeared on the X-Files. He has also appeared in two versions of Law & Order, and in episodes of Arrested Development, The Beat, The Wire, and Sesame Street.1
Law & Order: Special Victims Unit once crossed over with The Practice, a show which has crossed over with Boston Public,Ally McBeal, and Gideon's Crossing.
Beyond that-- a patient from The Bob Newhart Show once appeared on St. Elsewhere, while some of the cast from St. Elsewhere did a cameo on Cheers, a show that also crosses over with Wings, Becker, and, of course, its spin-off, Frasier, the cast of which once crossed into Caroline in the City which has crossed with Friends. Friends, meanwhile, crosses with Mad About You.2 The Cheers gang (no cheating here: the actors were playing their characters from the show) met Mickey Mouse (the helium-voiced Mouse, not the Guy in the costume) on a Walt Disney special, which crosses them over with those animated beings. An animated version of the Cheers gang also appeared on The Simpsons, which arguably crosses them with pretty much everyone else. But Scully and Mulder from the X-Files (which crossed over with Strange Luck and, through the character of Jose Chung, Millennium) have already also appeared on The Simpsons (in an episode which featured cameos by Chewbacca, Marvin the Martian, Gort, and Leonard Nimoy). Homer Simpson and characters from King of the Hill have made brief cameos on Family Guy. King...'s Hank Hill, like Daria's titular character, first appeared on Beavis and Butt-head. Although the spin-off shows went in very different directions, all can be legitimately connected.
Various chains link the characters already mentioned with an assortment of comic book superheroes.
Marvel and DC’s comic-book heroes have frequently met in crossover comic specials. Some of these present the two corporate entities as alternate universes, while others show the characters as simply co-existing (in the second Superman/Spider-man team-up from the 1970s, Spidey wonders at having only just met Wonder Woman, who lived in New York for many years. "New York's a big city," he muses). In particular, the Batman/Punisher team-ups show these characters as being part of the same universe.
Memorably and hilariously, the Punisher met Archie and the Gang when he tracked a suspect to Riverdale in 1994's Archie Meets the Punisher. Here, Riverdale is on the same Greyhound route as Gotham City, and Josie and the Pussycats make a cameo appearance.
Josie began life an in-house female rip-off of "Archie." Though they did not cross over as casually as other Archie characters, the Pussycats do occasionally show up in Archie's world. And Scooby-doo and the eternal teens of Mystery, Inc. have met both the Pussy-cats and Batman and Robin.
And the Three Stooges, Cass Elliot, the Harlem Globetrotters..... All of this happened on Scooby's first spin-off, The New Scooby-Doo Movies (1972-74). Each week, the gang would have an hour to solve the usual improbable mystery with the help of a meddling celebrity, with whom they would just happen to cross paths.
The Harlem Globetrotters also appeared in an early 1980s Gilligan's Island reunion movie. The cast of Gilligan's Island are explicitly referred to, along with characters from Magnum P.I. as sharing a universe with the characters from the second Brady Bunch movie. The first film, of course, showed the Partridge Family bus and featured the Monkees.
One could argue that doesn't count; the Brady movies were satiric, and not the real Bunch. In any case, the Brady Bunch also folds in through Superman and Wonder Woman, who made guest appearances in episodes of the show's animated spin-off, The Brady Kids. The Lone Ranger and Tonto also turned up in that series. The Green Hornet (who has teamed with Batman) is a blood-relative of the Lone Ranger.
The Bunch never met Mystery, Inc, though Scooby met nearly every existing Hanna-Barbara character in Scooby's Laff-A-Lympics, later renamed Scooby's All-Stars. I only know this show as a reference; I'm told it didn't feature the other members of Mystery, Inc. and that the characters, according to an acquaintance, "appeared as cartoon characters," whatever that means. In any case, Hanna-Barbara's Limited Animation Universe folds in with the rest here. , however, have linked to a variety of other characters. A version of Bewitched's Samantha and Darren, voiced by Elizabeth Montgomery and Dick York and using their series names (no "rock" or "stone" added), moved into the neighborhood during the original series. (Montgomery also made a gag cameo as her Bewitched character in How To Stuff a Wild Bikini, thus drawing in the Beach Party movies). The Flintstones Comedy Show (1980-1982), meanwhile, paired Fred and Barney, for no good reason, with Al Capp's Shmoo. Curiously, the Shmoo had appeared earlier in Fred and Barney Meet the Shmoo, in which its adventures had been quite separate from Fred and Barney's. In any case, this is one route by which all crosses with Al Capp's Li'l Abner, where the Shmoo's sad, satiric story was first told.
Of course, the comic covers of the 1930s-- when most comics reprinted newspaper strips-- often featured scenarios involving many characters from different strips. Tip Top #1 showed a boxing match between Li'l Abner and Tarzan, with various other strip characters watching. Between those covers and various strip crossover events: Sam's Strip's 1962 celebration of International Comics Week, the April 1, 1997 strip crossover event, and Blondie and Dagwood's 75th Anniversary Party, we can pretty much connect any two comic strip characters.
The late 70s also saw Fred and Barney meet the Thing, which kept Ben (the Thing) Grimm's adventures clear of the Bedrock segments. However, transitional material had the Marvel superhero dancing with Flintstone and Rubble, and this could be counted as a crossover of sorts. On both the Laff-A-Lympics and the Scooby-Doo Movies, the gang met the Blue Falcon and Dyno-Mutt, who much later crossed into Dexter's Lab
In the early 1980s, a new Scooby series was proposed, which would have featured Fonzie from Happy Days. It never materialized, but its pre-production work brings those characters (if we tweak the rules a bit) into our happy Crossover Heaven.... or Hell. Happy Days, of course, spawned a number of spin-offs, including Laverne and Shirley, Joannie Loves Chachi, Mork and Mindy, Blanski's Beauties, and Out of the Blue, most about as memorable as the later episodes of the parent show, and all sharing a common universe.
Out of the Blue featured a do-gooder angel and so, on that point, let's return to superheroes.
The X-Men have time-traveled into the future and met two different crews of two of Star Trek's Enterprises. The original Enterprise crew, meanwhile, met Larry Niven's Kzinti in an episode of the Animated Series, and therefore link with Niven's Known Space and Ringworld series. The NextGen cast take us to the DCU through a licensed crossover with the Legion of Superheroes. The ill-fated 2001 revival of Sid and Marty Krofft's Electra Woman and Dyna Girl placed the 70s camp heroes in the DC Universe.
Thor and other Marvel characters met Godzilla during the Japanese monster's tenure as a Marvel character. Godzilla has met nearly every other Japanese Giant Monster, and fought a version of King Kong.
Spider-man once met the Transformers. The Transformers once teamed up with G.I. Joe.
Several Marvel superheroes have met Capcom characters in a licensed videogame.
The "edgy" heroes of Gen 12 have experienced many crossover adventures, and met Marvel heroes, Superman, the Bone cousins, and Archie's Riverdale gang.
The Teenage Mutant Ninja Turtles have visited Riverdale. One of the turtles met Bugs Bunny and Winnie the Pooh in an anti-drug TV special, while the entire group, early in the career (pre-mass-mass-marketing to five-year-olds) crossed with Cerebus the Aardvark. Cerebus has met the Flaming Carrot, who has met the Ninja Turtles and Herbie the Fat Fury in his own comic. Since the Flaming Carrot belongs to the Mystery Men (though he did not appear in the film adaptation), they also connect. The Fat Fury crossed over with American Comics' "serious" super-heroes, and therefore they join their better-known DC and Marvel kin, and everyone else besides.
Plastic Man has become a part of DC's universe and met most of their characters, but he started out at Quality Comics. While there, he met Will Eisners The Spirit a few times, though these were limited to the cover of Police Comics. Their respective worlds were as different as Archie's and the Punisher's; nevertheless, one of their meetings clearly has the pair working a case together. In the twenty-first century, DC acquired the rights to the Spirit. Although they generally keep him in his own world, they once teamed him with Batman.
Superman and Batman have both met the aliens from Alien and Predator, who have (in Dark Horse Comics) fought each other. The Predator aliens, meanwhile, have duked it out with Magnus, Robot Fighter. Robot Figher, meanwhile, has met Turok, Son of Stone and the Man of the Atom when the heroes of defunct Gold Key/Whitman were revived in the 1980s.
Celebrities often made cameo appearances on the 1960s Batman show, when the Caped Crusaders climbed buildings. Sometimes, the celebrities appeared as their characters, creating a crossover. Lurch from the Addams Family was one such guest (but then, Scooby-Doo met both the Adamses and Batman), as was, inexplicably, Colonel Klink from Hogan's Heroes. Why a time-warped Nazi would be wandering around Gotham City is anybody's guess, but there he is, looking for escaped prisoners.
Colonel Hogan was mentioned, more logically, as an actual historical figure (as opposed to a TV character, which would not result in a crossover), on an episode of Green Acres, a show which crossed expressly with Petticoat Junction and The Beverly Hillbillies.
Even stranger is the fact that Superman has teamed up with a number of fluffy bunnies.
No, really. In the year 2000, the DC heroes crossed over to another universe where Bugs Bunny and the other Warner Brothers cartoon characters exist. The premise, alas, was largely wasted there, but better-handled in a 2003 animated special, The Green Loontern which teamed Daffy Duck and the Green Lantern Corps. Supes has also met the Nestle Quik Bunny in a 1980s rare promo comic, been rescued by Hoppy the Marvel Bunny in a mainstream DC comic, and he appeared in the first issue of Captain Carrot and the Zoo Crew. Presumably inspired by the success of Marvel's Howard the Duck (the 70s cult comic, and not the Lucasfilm adaptation which (all together now) laid a really big egg), "Captain Carrot" was set in an alternate universe where cartoony funny animals ruled. Later, these characters appeared in the bizarre Oz-Wonderland Wars comic, meeting the characters from L. Frank Baum's Wizard of Oz and Lewis Carroll's Alice in Wonderland.
Of course, Bugs Bunny and Daffy Duck met the Disney characters in Who Framed Roger Rabbit, which loops them back in via Mickey Mouse to Cheers. ...Roger Rabbit also features Betty Boop, who met a great many characters from newspaper strips in her original career. Bugs also met the U.S. Army's Private Snafu in at least one of those legendary World War II training films, while the Coyote and the Road Runner turned up in an episode of Night Court. The Warner Brothers animals once appeared on a TV special with the "Groovy Goolies," who, on their early 70s show, crossed expressly into Archie's Riverdale. Sabrina, the Teenage Witch, was a cousin to the Goolies and often appeared on their show; Archie and her other high school friends did so on occasion. For her 1990s TV incarnation, Sabrina moved from Riverdale, and also crossed over with You Wish, Boy Meets World, and Teen Angel.
An assortment of Disney characters meet the warriors of Final Fantasy in the licensed videogame, Kingdom Hearts.
In the 1980s, Batman and Elongated Man met the aging Sherlock Holmes.
Spider-man and other Marvel characters have met Dracula; Bram Stoker's vampire entered the Marvel Universe in the 1970s. Dracula, in other comics, met Zorro. In the movies, he has met Frankenstein's Monster, the Wolf Man, and Abbott and Costello, who have met a number of other characters, including the Invisible Man. Buffy the Vampire Slayer met Dracula in her fifth season, and characters from Buffy passed by Doctor Who in an issue of the "Season Eight" comic book.
Speaking of vampires: Barnabas Collins and the cast of Dark Shadows had a life beyond the cult 60s/early 1970s TV series, in a Gold Key comic. That comic featured an unlicensed, but very clear (the artists and writers were having some fun) crossover in #34 with Marvel's Dr. Strange (second series, #1-5). That series also featured an unlicensed appearance by DC's Green Lantern, represented by his hand and ring reaching up from under a table at a tea-party (he'd been drinking yellow tea. Yellow is his weakness). Barnabas also met Vampirella in a licensed crossover, and she has crossed over with characters from Batman in Catwoman/Vampirella: The Furies.
Giles, Buffy's mentor, implicitly refers to H.P. Lovecraft's Cthulhu Mythos in the Vampire Slayer's pilot episode. But the Frankenstein Monster already encountered Lovecraft's Old Ones, along with worlds and characters created by Herman Melville, Edgar Allan Poe, Jules Verne and Edgar Rice Burroughs, in Steven Utley and Howard Waldrop's "Black as the Pit, from Pole to Pole." The Real Ghostbusters cartoon crossed into Lovecraft's realm with the episode, "The Collect Call of Cthulhu."
Of course, Lovecraft and other writers, including Robert Bloch and Robert E. Howard regularly referenced each other's worlds. Conan the Barbarian apparently shares a universe with Cthulhu and company. It's not a consistent shared universe, but Lovecraft's influential mythos was never entirely consistent anyway. Lovecraft's ship Miskatonic also makes a brief appearance in Michael Chabon's Pulitzer prize winning The Amazing Adventures of Kavalier and Clay.
The League of Extraordinary Gentlemen crossed several characters from Victorian and eighteenth century fiction, from Holmes and Quartermain to Fu Manchu and Miss Flaybum and H.G. Wells's Selenites and Martians. The comic, at least, is set in some kind of alternate steampunk universe. The film connects fewer worlds (though it throws in an incorrectly-aged Tom Sawyer), but is set in a version of our earth. The third volume.... Thanks to The Black Dossier, it may be possible to link the League to most of twentieth-century English literature, French pulps, German expressionist cinema, and several comic strips. The League's second comic adventure includes characters created by Edgar Rice Burroughs.
Batman has met Burrough's Tarzan. Tarzan, as Lord Greystoke, appears in Robert Anton Wilson's Masks of the Illuminati, which features historical figures, including Albert Einstein, James Joyce, and Aleister Crowley.
Tarzan, Joyce's characters from Ulysses, and much of late-nineteenth/ early twentieth-century pulp fiction was crossed over by Philip Jose Farmer in the Wold-Newton Genealogy . This includes Fu Manchu, who, independent of Farmer, met Superman in an early issue and Spider-man in the 1970s.
And so on. By the turn of the millennium, any characters who have not met in licensed crossovers know each other (frequently in the Biblical sense) through online crossover fanfic.
A variation of this article, by this author, first appeared as "Crossover Feedback Loop, or the Six Degrees of Scooby-Doo" at http://crossover.bureau42.com/x.html
1. Wikipedia states that Richard Belzer's Detective Munch holds a record for the greatest number of appearances by an actor as the same character in different series.
2. Thanks to mkb for pointing out the Friends/Mad About You cross, through Phoebe's sister.