"Oh, no — another fan with ideas..."—Sokka's Actor, Avatar: The Last Airbender.
Ultimately, Canon is much smaller than the people who throw the term around like to think it is. Canon is limited to that which has actually been described in the source material. Especially in groups of writers, it boils down to what the writers specifically need to worry about for the purposes of the ongoing plot.
Fanon is the set of theories based on that material which, while they generally seem to be the "obvious" or "only" interpretation of canonical fact, are not actually part of the canon. Occasionally, the explanation seems good enough to just be "common sense." The salient point to remember is that when someone shouts, "That episode was terrible because it violates canon!", they are very often totally incorrect.
Fanon fills in holes that the writers may have deliberately left in order to have fodder for later stories. In addition to arising from a point of vagueness in the canon, Fanon can come into existence as a fact gained from a popular but non-canonical source, or taken from a different Adaptation. Because many fans mistake their own Fanon for actual Canon, they tend to get riled up when a new fact is introduced which does not literally contradict anything canonical, but invalidates what were formerly the most obvious assumptions. Many examples of Retcon and Continuity Drift that are imagined to be violations of Canon really only explicitly contradict Fanon.
Since many creators in the aftermarket series universe are fans, Fanon often shows up there, and if those creators in turn start writing for the actual show, Fanon may actually become Canon. Alternatively, you just have Memetic Mutation within the fandom.
Fanon often also refers to the body of information provided by otherwise-official sources. Television and movie scripts are a continuing source of fanon material — Captain James Kirk, for example, had a middle initial ("T.")...but his actual middle name ("Tiberius") was originally revealed in an episode of the Trek animated series; since that show's canonicity is debatable, it was considered "fanon" until revealed canonically in the 6th Star Trek movie. Note that this usage blurs the line between fanon and deuterocanon, though.
Warning: Fanon and accusations of Fanon are a classic Internet Backdraft, with the accusation commonly leveled by fans that have a different interpretation of the material — even when their theory is just as vulnerable to Schrödinger's Gun.
Compare Broad Strokes, where the events of a story are referenced in passing without taking everything said and done as having "officially" happened, and Headcanon , where the unofficial material is known and admitted to be unofficial, but personally satisfying. If the fanon was repeatedly hinted at by writers until it became fanon, but never actually confirmed in canon, it's Writer-Induced Fanon. See also Fandom-Specific Plot. Not to be confused with this Fanon or the Pope's robe.
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