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 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 or include 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.
Shared Universes, being effectively a feedback cycle of adaptations, are prone to having such problems growing and more obvious. Inclusion of elements explicitly or implicitly contradicting each other outright in "canon" materials and varying opinions on which parts should be accepted becomes common. First, the quality of writing varies greatly in the first place - and once more authors jump the wagon, this becomes pretty much inevitable. Second, usually the publishers have the final word while living in their own world, which rarely intersects with that of their authors or fans. Practically, this makes the decision-making either random, based on listening to their own ads (including astroturf) or delegated to self-proclaimed "experts" - the latter being either "independent" (that is, opinionated, but utterly clueless of the matter at hand), or already involved (which is prone to create Running the Asylum situation and/or tie in the internal conflicts). And then there's company politics. Even in those rare cases when the ultimate decision-making is retained by one of the project's leading authors, a big project is still likely to have authors working for their own purpose and some internal politicking - favours, appeasement, personal relations, prima donna hissyfits, etc - which all compromises both quality and clarity. The result is that having but one clear voice whom everyone heeds, however desirable, often is not feasible.
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 All The 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.
Contrast Headcanon, which fans know isn't official, but like to think is anyway.
- 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.)
- Often after a major retcon or reboot, classic stories are considered canon until proven otherwise by new canon. Birthright was considered Superman's origin story even after Infinite Crisis until Johns wrote Secret Origin.
Film - Animated
- Titan A.E. had two short novels that came out with the film, to help explain the two main characters' pasts and motivations, as well as the world in which it is set.
Film - Live-Action
- The Star Wars canon is explictly patterned after Star Trek canon. However, since the Star Wars Expanded Universe has its own canon hierarchy, most people don't realize that and 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).
- Lucasfilm has Leeland Chee, whose entire job is "keeper of the canon": to draft the canon policy, keep the EU material consistent with Lucas' story and vision of The Verse, and in the case of any contradictions of events between sources, to come up with the official canonical explanation. So while Lucas does not view the EU as part of his story, the official policy is that the EU is canon to the Star Wars universe.
- However, after Lucasfilm got bought by Disney and the new trilogy was announced, all of the old Expanded Universe material was swept aside into the not-really-canon Star Wars Legends label. The only canonical materials as of 2016 are the films, Star Wars: The Clone Wars, and Star Wars Rebels (i.e. the CG animated series).
- 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 HP Lovecraft, but sometimes also the work of August Derleth. Fans argue, especially with the changes in character Derleth created.
- There's some argument over what is and what isn't canon in the works of JRR Tolkien as they relate to Middle-Earth; he made many, MANY changes to his works over the course of his lifetime. A book (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.
- 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.
- 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.
- 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.
- In terms of Buffy the Vampire Slayer, in addition to the seven seasons of the TV show, all the Season 8 and 9 comics have been declared official canon by Joss Whedon.
- All other Dark Horse comics produced before the Season 8+ comics, however, are not considered canon.
- Transformers, with all its spin-offs, is a massive canon snarl. Generation 1 and 2, Beast Wars and Beast Machines are the main canon, sort of, but there's also Robots in Disguise, the Unicron Trilogy, the live-action movies and the new Transformers Animated. Linnaean taxonomy has nothing on Transformers continuity families of multiple micro-continuities, including conflicting stuff like toys' "tech spec" bios and the kiddie cartoon shows. And then the "Universe" comics seem to have made it all a Marvel/DC-style multiverse, where characters pop in and out of continuities with alarming frequency.
- Sakamoto outright stated that the Metroid Prime series devoleped by American dev Retro Studios was not Canon, despite there being seemingly no problems with it in terms of canon.
- It was because he got butthurt people liked the prime games more then his awful ideals of what the Metroid series should be.
- Many games (and especially Visual Novels) have the problem of the story branching into Multiple Endings, thus creating a number of mutually exclusive but canonical happenings. This becomes particularly relevant when the source material is adapted to a linear medium like a TV series and one of the paths has to be chosen, adding "extra canonicity" to it. The same applies to sequels. Choose wrong, and the original fans will be up in arms; and there likely is no right answer. See Tsukihime for an example.
- 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.
- 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.
- 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.
- In The Legend of Zelda, all 16 games are canonical, (and not the games by Phillips) albeit in three Alternate Timelines that diverge at Ocarina of Time, according to the 25th anniversary encyclopedia Hyrule Historia. These games include multiple people named Link and Zelda (about ten each).
- 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 Fairly OddParents Live-Action movie A 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.
- "A cannon can be used to reinforce canon"