|See also:||fanon, retcon, Word of God, Endgame, Canon Nazi|
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Canon (in the context of fandom) is a source, or sources, considered authoritative by the fannish community. In other words, canon is what fans agree "actually" happened in a film, television show, novel, comic book, or concert tour. The term derives from the theological concept of canon, the foundational texts of a religion.
Specific sources considered canon may vary even within a specific fandom.
Sherlock Holmes fandom was the first to use the word "canon" in its fannish sense. In his 1911 essay "Studies in the Literature of Sherlock Holmes," Ronald Knox jokingly compares the Sherlock Holmes stories to the Bible. Early Holmes fandom quickly took to referring to the collected works as the "Canon" or the "Sacred Writings."
The word "cannon" refers to a form of artillery gun. Many fans have limited exposure to the religous usage, and use the spellings interchangeably, which leads to more or less amused rants by those who do understand the difference
Official vs. Canonical
In some cases, a fandom may embrace one work by an author, and yet reject a closely related work as "not canon." An author's works may be considered canon, while statements by the author are often considered to be merely opinion.
Prop canon is related to physical objects shown in a movie or a TV show, for example college degrees, driver's licenses, etc. Lacking conflicting evidence, prop canon is considered a reasonably canonical source, but occasionally dialog, or later show development may cancel it out. For example, in Stargate SG-1, Jack O'Neill had different birthdates depending on which driver's license was shown, while Rodney McKay on Stargate Atlantis has a certificate on his wall that gives his name as 'Rodney Ingram McKay' which was later contradicted in the episode McKay and Mrs. Miller when his sister said his full name was Meredith Rodney McKay.
Grey canon or gray canon refers to canon that is ambiguous in its canonicity: a detail implied or easily inferred in canon but not confirmed outright; a detail brought up in a creator commentary or podcast but never stated in canon itself. (Essentially, the canonicity of that fact is in a gray area.) An example of grey canon would be the "Previously" segments in Battlestar Galactica, which are not, in fact, previously aired material from an episode. Since the not-actually-Previouslies do not occur within the bounds of an episode, it is not unreasonable to call them non-canonical; but since they affect the episode to which they are attached, it might also make sense to call them part of canon. Another example is the character Billy from Power Rangers, whose surname was never stated on the show but who was referred to on some merchandise (and, years later, on the official Power Rangers website) as "Billy Cranston." Although many fans have embraced "Cranston" as canonical, there are some who do not accept it as canon.
Closed vs. Open
A fandom is considered to possess a closed canon when no additional source material is expected to be forthcoming (there are no upcoming books, episodes, or movies in the series). Examples of fandoms with closed canons are Due South, P.G. Wodehouse, and Good Omens.
Fans of a universe who find it extended in a different medium (as comics following a completed television series, television series following a movie, etc.) sometimes have difficulty agreeing on what constitutes canon. There are Star Trek fans who only believe the first series (ST:TOS) is canon and Homicide: Life on the Streets and Wiseguy fans who deny the "canonicity" of their post-series TV movies. Certainly there are plenty of Star Wars fans who consider Star Wars a closed canon, even though tie-in books are still being published. In some cases, fans look to creators for guidance (e.g., canon for The Sarah Connor Chronicles officially ignores the third movie in the Terminator universe); in others, a rough consensus emerges (e.g., Supernatural comics are generally rejected from canon due to a high continuity-error rate). In still other fandoms, no consensus exists, and creators of fanworks may disclaim extensively exactly what they are and are not willing to consider. This happens a lot in comics fandoms (e.g. DC Comics, X-Men) where canon has been ongoing for decades, frequently retconing or contradicting itself; writers will sometimes simply pick the characters they want to write regardless of whether those characters were ever all on the same team (or all alive) at the same time.
Canon in Real Person Fiction
RPF has inherited the story tropes and fanspeak of FPF, and along with the woobies and angst and all those first times came a way of discussing and of conceiving of RPF as a form of fiction. The terms canon, fanon and AU get bandied about in RPF circles the same way they're used in FPF meta.
While canon in FPF usually refers to the single recognized source for the fandom, in RPF there is no single source, and sources often contradict. Not surprisingly, fans don't agree about what is and is not canon in RPF fandoms, or how important it is to a story that isn't represented as an AU to get all the facts right. All RPF is a balance between the Real and the Fiction, but not all RPF sets the same balance.
Writing canon-based RPF has been compared to writing historical fiction where an author is "filling in the blanks of an event that really happened". This kind of story encompasses everything from Sports RPF stories that contain play by play recreations of actual games to Historical RPF which recreates the lives of sometimes long dead historical figures, extrapolating from detailed knowledge and research of the period. Actor and musician RPF also has these sorts of stories where the filming of specific episodes of television shows, parts of films or actual concerts are an integral part of the plot.
The level to which an RPF story is not canon compliant, and the point at which it becomes an AU, is open to debate. At the extreme end of the spectrum, stories that place real people in scenarios totally removed from their actual lives, stories where the members of My Chemical Romance are on a spaceship or where J2 are bullriders are obvious uses of the real characters in completely non-canon settings. While some aspects of their canon selves might come along for the ride, no one thinks the story is supposed to represent their real lives.
The less obviously non canon-compliant story is the one where My Chemical Romance are still musicians and J2 are still actors, perhaps even doing many things the real people have actually done, but the desire on the part of the author for perfect detail, or their willingness to use non-celebrities in supporting roles, may push the story away from true canon compliance. This distinction is harder to draw, and different fandoms have different cultures that determine where that line is.
To some extent all RPF, and all historical fiction as well, contains elements of fiction mixed in with the reality of the characters. In RPF though, the author may be setting the story in a simulacrum of the person's real milieu--describing some semblance of a television show studio or tour bus--but was never intending to realistically portray that person's life. In these stories, the use of fanon in place of canon is often intentional. In other cases the author chooses not to learn the difference between the two, either because they don't want to research every detail of their characters real life progenitors, or they don't think they should.
Canon in Pop Culture
T-Rex of Dinosaur Comics once proclaimed several events in his life to be non-canon, including "any event in which I was embarrassed." 
Mistaking Differing Characterizations For Non-Canon
Some fans feel that canon also extends to characterization and not just the facts or events that take place in the published novel or on screen. The two are so intertwined that a character who behaves "non-canonically" is also considered to be based on "illogical characterizations" that are not supported by the source:
- "For my own part, I initially was somewhat startled to realize, upon reading my first fanzine those many decades ago, that other people could and did interpret these characters somewhat differently than I did. After a while I concluded that there was a continuum of logical characterization--always demonstrably rooted in canon--along with the who-the-fuck-*are*-these-people "interpretations." As far as I can tell, these are based on projection, wish fulfillment, and/or what I call "spackletext" (as opposed to genuine subtext) from writers (well, story posters, at least) who want the characters to meet their specific desires in emotional/physical/therapeutic gratification and are completely oblivious or simply uncaring as to how that is achieved." 
- copperbadge etymological discussion of canon vs cannon accessed 2013-04-17
- emily_shore Two kinds of RPF, posted February 11, 2009, accessed March 25, 2010
- mrs_leary in bradleycolin--Crossing the line in RPS..., posted September 14, 2009, accessed March 25, 2010
- T-Rex oh man, last night was so non-canon April 06 2004. Accessed October 4, 2008.
- klangley56 commenting in Will the real characterization please stand up?, dated May 29, 2007; accessed Feb 8, 2011; WebCite.