That which counts, in terms of continuity.
Canon, as it applies to television series, is substantially different from its literary counterpart. For example, there is no question of which Sherlock Holmes stories (the first literary works to which the term was applied) are canonical: those written by Doyle are, everything else isn't.
Television canon works much differently, as there are many authors involved. Works not officially sanctioned are generally outside of canon, but what remains inside is more nebulous. Officially licensed material, novelizations and tie-in novels are not usually considered canon. Even broadcast material can be excluded from the canon when decreed by Word Of God.
The primary issue is that canons for completed works (especially with a single author) are descriptive, whereas fans' attempts to define canon for ongoing works are prescriptive. If a fact is "canon", you are "not allowed" to contradict it.
The concept of canon is almost entirely an invention of fandom. The writers will ignore, include, or change whatever facts they damned well like. This is not to say that the writers totally lack a sense of continuity, but it is a much weaker concept than "canon" as presented by fan communities. Writers can tweak continuity quite a lot without actually breaking it by using Broad Strokes.
In fan communities based on very loose continuities, "canon" can sometimes boil down to "the bits we like". Fans will attempt to find any excuse to "de-canonize" facts that they personally find inconvenient.
A related term is Deuterocanon (known here on TV Tropes as Word of Dante), which in this context refers to those persons, places and/or events which are not explicitly shown on-screen, but which are considered "official" or close to it. For canon that comes not from the source material but from pronouncements by the creator, see Word Of God. For the contrary idea that something is canon only if it appears in the source material (external opinions of the creator not included), see Death of the Author.
This concept is related to the literary term used to describe a body of work that is considered the foremost in quality and significance. For example, if one refers to the English-language literary canon, it is understood that one is speaking of books such as A Tale of Two Cities by Charles Dickens, Moby-Dick by Herman Melville, Pride and Prejudice by Jane Austen, and Heart of Darkness by Joseph Conrad—in other words, most of the books you read in High School are part of the English-language canon.
Canon should not be confused with Fanon, but everyone does it all the time. See Fanon Discontinuity for when people decide en masse to disregard actual canon, and Canon Discontinuity when the writers do it. Alternatively, see the Continuity Tropes index for all related concepts. Official Fan Submitted Content is when the creators ask the fans to add to the canon.
For the similarly titled Visual Novel, see Kanon. Not to be confused with the camera company of the same name, or with singer/songwriter K'naan. Or Ganon.
When it comes to the Gundam franchise, the official word from Sunrise is that all works that appeared in official releases count as canon unless stated otherwise. Even if they try their hardest to line up with continuity and get appearances in crossover media like Super Robot Wars, this still creates quite a lot of problems. Most of the time they act more like written guides are most canon, anime is more of a film/movie adaptation of what actually happened, yet much more canon than manga and novels, and games are, all non-canon, unless retconned by any previous mentioned media and with no contradiction with the guides.
Superhero comics have wildly fluctuating levels of canon with generally the most popular stories written by currently established writers being considered canon, often even if they weren't originally.
For example, Kingdom Come, originally an Elseworlds story, was eventually retconned to be the official future of the DC Universe (and later retconned to be one of the Fifty Two earths with the Superman of that universe interacting with his mainstream universe counterpart.)
The Star Wars canon is explictlypatternedafterStar Trek canon. That is, "real" canon is just the movies and the Clone Wars cartoons, and that while the Expanded Universe material has its own internal continuity and canon, it is most assuredly not the "official canon" of Star Wars. This policy has been reinforced by Disney, the new owners of the franchise. However, most fans don't seem to realize that there is a difference, or else deny the difference matters, or simply declare that the owners of the franchise have no say in the matter and thus consider EU canon as the Star Wars canon. The struggles to retcon EU to accommodate new Clone Wars material has actually caused one SW EU author to quit. For more on this, see Expanded Universe (and the Star Wars Expanded Universe).
Dragonlance fans regard the stories written by Margaret Weis and/or Tracy Hickman as being the official canon, but attitudes towards the books written by other authors range widely.
The Cthulhu Mythos canon is sometimes only the work of H. P. Lovecraft, but sometimes also the work of August Derleth. Fans argue, especially with the changes in character Derleth created.
In fairness, the mythos is all about horrors beyond our comprehension, so its natural that different writers would have different interpretations of the material.
There's some argument over what is and what isn't canon in the works of J. R. R. Tolkien as they relate to Middle-Earth; he made many, MANY changes to his works over the course of his lifetime. The Hobbit is sometimes considered non-canon because it was not originally created as part of Middle-Earth, despite the fact that the widest-known book in the setting, The Lord of the Rings, was meant as a sequel to it.
Literature/The Silmarillion and the Appendices to Lord of the Rings are accepted as the most canonical accounts of the pre-Hobbit history of Middle Earth but for events not covered in them one has to delve into writings unpublished during Tolkien's lifetime, which are much less organized. The History of Middle Earth series published by his son is 12 volumes devoted to documenting the evolution of Tolkien's ideas and manuscripts and STILL didn't exhaust the known body of manuscripts left behind at his death.
This is further complicated by the fact that Tolkien was himself a philologist and wrote a complex Literary Agent Hypothesis into canon explaining how he obtained a manuscript of the Red Book of Westmarch, Frodo and Bilbo's first-hand acount of their journeys. Tolkien's preferred way of dealing with apparent inconsistencies in canon was to attribute them to a story having been handed down in more than one form before reaching his ears or to the personal biases of those involved in the transmission of the story.
In Sherlock Holmes fandom, the original works penned by Sir Arthur Conan Doyle are the primary canon, with different spinoff 'verses having their own subsidiary canons. However, what facts are and are not canonical is made less clear by the trope-naming Literary Agent Hypothesis, where Doyle is merely John Watson's literary agent and the stories are all first-person accounts penned by Watson (and in a few abberent cases by Holmes himself). Watson refers repeatedly to Doyle editing parts of his stories—and in turn Holmes regularly accuses Watson of "re-imagining" cases to be more exciting and trope-tastic—but Watson also edits himself, alluding to cases still too dangerous or controversial to publish. There even seems to be some Retcon involved around Moriarty, with stories published after "The Final Problem" and "The Adventure of the Empty House" suggesting that Watson WAS involved in the long cat-and-mouse game leading up to the destruction of Moriarty's crime network but suppressed that fact in order to protect the ongoing investigation.
There are actually fans of the Harry Potter novels who disagree with some of the information J. K. Rowling has released since the publication of the last novel, Harry Potter and the Deathly Hallows, to the point of openly stating that Harry Potter doesn't belong to just Rowling anymore, and if the author wants to add stuff, she'd better write another book. They are so vehement about this that one of them even tried to dictate this "policy" to Rowling herself. Rowling laughed in the fan's face.
Doctor Who has no official policy from above on what is or isn't canon. Being a show about time travel and history being altered, this probably makes sense.
"Why all this fuss about canon - and, indeed, continuity - in a show about a man who changes history for a living?" Steven Moffat (link)
Paramount maintains that nothing that didn't happen or wasn't referenced onscreen in Star Trek is canon. This technically includes the film series beginning with the 2009 "reboot", which features a few characters from after Star Trek Nemesis in the "prime universe". Star Trek The Animated Series is generally not considered canon (with the possible exception of the episode "Yesteryear", according to the authors of The Star Trek Encyclopedia). The official status does seem to change from year to year, considering how many writers worked on both that show and the original series.
Currently Paramount's policy is that canon consists of the movies, the live-action shows, and TAS. With the only possible exception being the Voyager episode "Threshold".
Like Star Wars, Babylon 5 also has canonical licensed tie-in media.
LOST's ARGs and tie-in video game have mixed canonicity, and the showrunners have used the podcast to declare what can be taken as canon and what cannot.
Per Word Of God, only the Peanuts comic strip counts as canon, not the animated TV specials, TV series and movies.
Religion, which is in fact the origin of this concept, also has its share of both Canon and Fanon. In addition to The Bible, for instance, Jews have The Talmud and many old Jewish legends besides, and Christians have works from various Jewish and Roman historians such as Josephus, Gnostic cults, and certain popular contemporary legends as well. Note that true believers do not necessarily automatically disregard all of these apocryphal works as wholly false; in fact, Jews and Christians will often borrow from these works to interpolate from the canon works when adapting various parts of the Bible to television and movies. They just don't require anyone to believe in these "supplementary" writings in order to be a believer.
Note that the Bible has been officially and permanently fixed since the 300's. Before this many smaller groups of religious sects argued over which Gospels were in fact true to the canon of Jesus. For example, you might be surprised to hear that there was a Gospel of Peter. It's not in your local Bible though.
Indeed, if the early compilers of the Bible had the information we do now, what became canon would probably have been very different. Just one example: early Christians believed that the John mentioned as Jesus' disciple, the author of the Gospel of John, the author of the 3 Letters of John and the author of Revelation were all the same person. They're now widely considered to have been at least three different people (most historians would still say that the Gospel and the Letters probably had the same author, but the other two are undeniably different), which casts doubts on whether it was right to include those books.
Ordinarily Wizards of the Coast takes the position that any video game adaptations aren't canon. But then the Forgotten Realms novel and sourcebook writers chose to make canonical several plot points from the BioWare games. For example, the whole Bhaalspawn plot from the Baldur's Gate series was referenced in the 3.5E sourcebook Lost Empires of Faerūn, and other material mentions the Wailing Death in Neverwinter Nights.
In Battle Tech, there is now 'divergent canon', thanks to Wiz Kids' improper seizure and use in 2001 of a FASA-era submission, which included an extensive history of the 'Eridani Light Horse' mercenary unit. Wiz Kids and the author arrived at a settlement whereby he provided a new version and it was treated as canon —- without the author signing over the rights in his contribution, the only known time this has occurred regarding official Battletech material. Topps later bought Wiz Kids and, after a few years, hijinks ensued, followed by a lawsuit. Ultimately the US 9th Circuit Court of Appeals affirmed that the submission is a contribution to part of the Battletech property. Many of its details have been contradicted by new canon material since its publication, but since the author still retains copyrights in his contribution, it effectively forms its own branch of Battletech canon which he has stated he intends to build upon at some point.
In the games Wing Commander III and Wing Commander IV, which also had novelizations contracted out by Origin, you are given several choices as to an action path to take, as part of the "interactive movie" feature of those games. Origin (later bought by Electronic Arts) has declared that the choices taken in the novels are the official history of the in-character universe. Sorry, Locanda IV.
Metal Gear Solid has two endings, one in which Snake's love interest Meryl dies and another in which she survives. Initially, the creators decided to handle the issue by simply ignoring it; Metal Gear Solid 2's story neither contradicts nor confirms either ending, making them both possible. It wasn't until the fourth game that we found out that Meryl lived.
Skyrim implies via Sheogorath's daedric quest that the canon version of the Champion of Cyrodiil was an Imperial male who joined the Thieves Guild and the Dark Brotherhood. Its notoriously Unpleasable Fanbase went nuts over the Champion not matching with their varied versions.
Castlevania has been subject to multiple canon revisions, first with series lead IGA cutting out certain stories from the canon, then later adding most of them back, then we get Lords of Shadow which ditched previous continuity altogether.
Fighting Games have their own problems when they introduce an actual narrative into the mix; usually they involve some kind of tournament or Big Bad that every single character (often more than a dozen!) is trying to triumph over, each with his or her own ending for doing so. When a sequel rolls around, it can be a Herculean task to figure out who won the previous game, which other characters had endings that could play out even if they didn't win, and which have been relegated to what-if scenarios.
Melty Blood, which is based on Tsukihime, actually gets around this by creating an imaginary story branch (The 'Satsuki' route).
This is especially a problem in games such as Tales Of Symphonia, where the game varies slightly by which character you choose as Soul Mate for the main character. And thus begin the Shipping Wars.
Pikmin. In the bad ending of Pikmin 1, Olimar fails in collecting all the ship parts and doesn't make it home. This obviously isn't canon because in Pikmin 2 he lands on Hocotate and it is requested that he go back.
This also happens in Fire Emblem: Radiant Dawn. The game assumes that you got the most perfect ending possible in the predecessor, Path of Radiance. This means that you would have had all possible characters recruited and alive, as well as having defeated The Black Knight, a boss battle you could escape. This makes less sense as Radiant Dawn offers you to transfer your game save from Path of Radiance to draw from it and alter things in the game. On the other hand, the story of Radiant Dawn would be somewhat boring if all characters had died in Path of Radiance.
The X-Universe series has about half a dozen novels set in it, and has an encyclopedia designed to be the official explanations for everything. Unfortunately, an apparent chronological error in said encyclopedia leaves a bit of confusion surrounding the Second Terraformer War.
The movie Ben 10 Alien Swarm was later made canon however with the episodes "Revenge of the Swarm" and "The Perfect Girlfriend" which acted as sequels to the movie and explained many unanswered questions.
The Fairly OddParentsLive-ActionmovieA Fairly Odd Movie: Grow Up, Timmy Turner! takes place in the future and shows that the Tootie/Timmy shippers won out in the end, as Timmy gives up his fairies for Tootie, but a loophole in the rules allows him to keep his fairies, so long as he uses them for unselfish purposes. Tootie also is allowed to learn of the fairies. Although the movie is not the finale of the series itself, it seems to set the events of the future in stone.