Fanon is any element that is widely accepted among fans, but has little or no basis in canon. Sometimes it's a small event in canon that gets exaggerated; sometimes it's something in a fanfic story that gets picked up on and repeated by other writers until it's so common that newbies might think it's a canonical fact.
The Origin of the Phrase
The precise date that "fanon" entered into fandom vocabulary has not been established. Discussions on the Virgule-L and Blake's 7 mailing lists over the spring and summer of 1993 used the phrases "fan canon" or "fan dogma" or "fan consensus" or "fan universe."In Feb 1993, Sandy Hereld posted to the Virgule-L mailing list:
"We had a Pros panel at Escapade last year that turned out like a game show. The moderator would say something like, "Doyle eats health food" and people would yell, "canon" or "fan canon" (and then argue it out--what episode if canon, or what early story if fan canon). It was great fun. Weird, how our additions to a fandom take on life of their own."
The Blake's 7 mailing list eventually decided to adopt the phrase "fan canon" which then led to a few fans objecting:
In support of the use of the phrase "fan canon":"Since we are talking about "fan canon", I really wish to put
my objections to the (mis)use of the word "canon" to describe what we are talking about. Taking my trusty dictionary in hand, I find: canon [noun]:
- 1. a Church decree enacted to regulate morals or religious practices.
- 2. (often pl.) a general rule or standard, as of judgement, morals etc.
- 3. (often pl.) a principle or accepted criterion applied in a branch of
- learning or art.
- 4. (R.C. Church) the complete list of canonized saints.
- 5. (R.C. Church) the prayer in the Mass in which the Host is concecrated.
- 6. a list of writings, esp. sacred writings, officially recognised as
- 7. a piece of music in which an extended melody in one part is imitated
- successively in one or more other parts.
- 8. a list of the works of an author that are accepted as authentic.
- 9. (formerly) a size of printer's type equal to 48 point.
Sorry, I put the whole lot there so you wouldn't think I was selectively editing for my benefit. I think we would agree that sense 6 (and maybe 8) are the ones that would apply to writings, such as fan fiction. If we call these themes "canon", that would immediately imply (quite strongly) that anything that didn't fit within these themes/assumptions/ characterisations (wot we are discussing) isn't "genuine" or "authentic" Blake's 7 writing. And that simply ISN'T so, and I would hate for us to imply this sort of exclusivity when we don't need to! So please, *please*, PLEASE stop using the word "canon" in this discussion!Recurring themes, or something, but not canon.
By August 1998, some 5 years later, the phrase "fanon" was being used by media fans on Usenet and in forums like Ci5 (The Professionals), Virgule-L and MUNCLE mailing lists. By October 1998, fanon was mentioned in a Friscon convention report on alt.startrek.creative.erotica.moderated. But the earliest known reference to the use of the word fanon was in April 1998 when Emily Salzfass used the word 'fanon' on alt.startrek.creative.erotica.moderated:"My own use of the terms Canon and fan canon are influenced
by Sherlock Holmes fandom (and Holmesians would probably flay me for using fandom to describe them 8-). Following their use of Canon and Apocrypha, I would apply them to B7 this way:
- Canon -- the series as aired.
- Apocrypha -- scripts; stories told by TN, CB, the actors and others involved in the production; novelizations. Essentially, stuff from those who are not fans.
- Fan canon -- ideas generated by fans, based on Canonical and Apocryphal sources, that are widely accepted among the community. Often these ideas form the basis of fan debate; 'is Avon pushing the sanity envelope' in the 4th season is fan canon IMO because the question is a frequent issue of debate and accepted as a valid matter for discussion by most fans whether their own answer is yes or no.
Fan canon is dynamic; it is always evolving. There is also substantial overlap among these categories. For instance, Blake is an engineer is Apocryphal under this scheme because it comes from a part of the script that didn't get on the air and was picked up in the Programme Guide and has now been accepted as a fact by most fans even when they don't know the source.These are all matters on which reasonable minds may differ."
Years later Emily wrote:"I am very "out" about my Trek passion, and the paper I'm writing suggests as much. I am all over saying that Trek fandom is a great community, that fanfic is an empowering and positive forum, and that we as a group are thoroughly and effectively shaping an inadequate canon to fit our growing needs. [I've also coined "fanon" to mean "fan canon"...it's yours, if you want it...]"
"I mean, of course I didn't actually invent this word. If nothing else, it's such an easy slushing together of words -- fan + canon -- that other folks probably stumbled across it just out of laziness, throughout the history of fans rewriting source material -- aka, since cave painting days."
A variation of fanon is "personal canon" or "headcanon," which is a set of "fanon"-like facts that are accepted as canon by an individual fan or a smaller group of fans, sometimes in the making of a shared universe.
When Fanon Becomes Canon
Sometimes fanon becomes canon because TPTB planned to reveal a canon fact all along, and fans merely beat them to the punch by figuring it out ahead of time.
Other times, bits of fanon are created by fans, then picked up by TPTB and made into canon. For instance, in Stargate SG-1 fandom, fanfic writers gave a recurring character, Major Davis, the first name "Paul." A few years later, the show's writers officially gave the character the first name Paul.
Similarly, in Highlander fandom, the fans quickly took to calling the unnamed U.S. city where Duncan MacLeod lived "Seacouver," because it was clearly meant to evoke the feel of Seattle while looking suspiciously like Vancouver (where the show was filmed). Eventually, the show creators officially adopted the name for the city, referring to it that way offscreen. (Although it was never mentioned on the show, it did appear as prop canon--a newspaper contained an ad for an event at the "Seacouver Community Center" -- and in some of the tie-in novels.)
As an Established Fan "Understanding"
When plot points and characteriztions become entrenched in a fandom, it can be difficult to separate out what is "canon" and what is not. Often fans are entirely unaware where a fanon element originated
One example from 1980 from the Star Trek: TOS fandom. Like Kraith and other popular fan-created universes, the Sahaj Universe spawned much fanon, and even the fans had a hard time keeping it straight. When a fan used a word that Leslye Lilker had created in a story, the fan and the editor of the zine it appeared in had to apologize. "In "And God Against All" In R&R XII, Collette Mak used the term, Vljn'd'Jt, which is a term coined by Leslye Lilker for the Sahaj universe. Collette thought it was one of the usual terms in Treklit. I knew it was Leslye's, but I missed It. Both Collette and I apologize; the term is Leslye's, and our use of it was unauthorized."  Another example, this one from the Blake's 7 fandom: in 1985 and 1986, there had been several instances of a fan named "Keith Black" selling bootlegged copies of Blake's 7 zines, one of them of the fic by Ann O'Neill called "Between Black and White" (see Between Black and White). This may have made fans extra-sensitive, including the producers of a zine called Mascarada, to any possible raised eyebrows regarding fanon, head canon, and canon. Susan R. Williams, the author of this zine, included this statement in the preface to the story "The Portage" in "Mascarada":
Even in newer fandoms where these origins may still be traceable. In November 1999, Laura Jacquez Valentine pointed out that:
AUTHOR'S ACKNOWLEDGMENT:The first person to suggest that Kerr Avon had a sensitized allergy to standard drugs used for purposes of interrogation was Ann O'Neill, in her excellent story, "Between Black and White" (Standard by Seven, Number 9). The idea made such good sense to me at the time that, to my embarrassment, I find I had unconsciously assumed it was part of the "background" established for Kerr Avon in the course of the aired episode. I understand that the idea was original with Ann O'Neill, who has graciously consented to lend the plot device to me for the limited purposes of this story and its eventual conclusion. I feel the richer for it, and wish to thank Ann O'Neill, as well as acknowledge the source of the idea.
One thing that's interesting about newer fandoms is that you can often peg these things. For example, the Sentinel canon vs. fanon page contains initial points for such things as Blair's unreliable car.
And in Star Wars fanfic, I'm personally responsible for the convention that Palpatine's first name is "Bob", and for Masters cutting and braiding their Padawan's hair and weaving their own strands into the braid ("Scissors", 09 Jun 1999).It's interesting to watch these things spring up. I bet most of the people who use the cutting/braiding bit of fanon now, only a few months later, have no idea where it came from. 
Controversies and Challenges
Some fans think fanon is a sign of lazy thinking or lazy writing, arguing that a fan writer should come up with her own story elements or interpretations, and not repeat the story elements or interpretations of others. For example Victoria P. writes on fanon: "After a while, fanonical characterizations all start to feel the same, and isn't one of the aims of fanfic to produce diversity? [...] I mean, when the fanon is so incestuous that the new writers can't tell fanon from canon and accept the former as the latter, that's a problem, to me." ' In a related vein, there are fans that believe that overwhelmingly popular fanon acts like a stranglehold on the range of possible character and story interpretations, essentially creating an orthodoxy from which authors stray at their peril.
In addition, reusing even basic phrases that have become fanon have, in the past, stirred debate. The 1984 column "The Protocol Droid," published in the letterzine Jundland, Too, contains a letter from a fan saying she was worried because her fanfiction was about the fall of the Jedi enclaves, but that she'd learned that concept of "enclaves" was from another fanwriter's universe. She wanted to know if she could still use this term. The answer? Absolutely not without specific permission from the fan originator, and if any similar appropriation was done by accident, the fan needed to apologize by writing a personal letter. Many fans today would not require permission in this situation as the writer is not reusing a specific story or a character, only a concept.
One fan admitted to being being tangled up in her own fanon as she read another fan's story. Maggie Nowakowska, author of ThousandWorlds, wrote in 1979: "... I thought the idea of using "Lucas" for the name of Luke's father a good idea ... wish I had thought of it. One problem I can see with future SW fanfic is the confusion of names for the same beings. I ran into this in a later piece when I found myself thinking, "No, Greedo's people are called 'Deseratines'" ... and I had to stop and remind myself that that name comes from the Thousand-Worlds series and that this author can call the creatures anything she damn well pleases. 
Occasionally a writer's use of fanon can indicate an unfamiliarity with the actual source, as if she is writing fanfic based not on the show, but on other fanwriters' interpretation of the show. For instance, in The Sentinel fandom, it was once quite common to find stories that depicted Blair Sandburg as a life-long vegetarian; use of this fanon element became widespread, despite the fact that several key episodes of the show (such as "Blind Man's Bluff" or "Spare Parts") indicate that Blair is, canonically, an enthusiastic omnivore. As writing fanfiction without having actually viewed the source is generally looked down upon, use of fanon that blatantly contradicts canon can be taken as a warning sign to readers that they are about to encounter badfic. Similarly, stories that contradict fanon (but not canon) may be described by some readers as OOC or AU, in cases where fanon has become so powerful that it attains a force equal to (or surpassing) canon itself.
However, not all fanon contradicts canon. Certain types of fanon are created because certain details or interpretations invented by fans seem so right, so true or plausible or pleasurable, that they're repeated by other writers almost as a form of tribute, as if that's what must have really happened. From this point of view, fanon is an essential aspect of how fanfiction writers improve on or flesh out (often sketchy) source texts.
Canon and the VCR
In the days before VCRs and/or easy availability of home viewing, fans of television or film fandoms had extra challenges; not everyone was on a level playing field regarding memory and access to reruns of their favorite shows.From a fan in 2000:
Actually, I can see both sides of this issue, especially when you're dealing with a show as canon-heavy as [ Starsky & Hutch ] has always been. Many fans writing now don't have the priviledge of having a library of uncut tapes from which to do their own research. And some things in SH *are* considered a given by fandom at large. We've all sat in fandom gatherings were stories were criticized because some critical issue in fanon wasn't used when the story called out for it, and it interfered with the believability of the story. I wrote my first SH story, Crystal Blue Persuasion, after having seen about 6 episodes (one of them the Shootout, from which the story takes off). My first readers had some problems with the story because of all the things I *didn't* know. I was able to edit it better with a few more episodes under my belt, so that it agreed more with canon and was more believable. (I'd come from Miami Vice fandom which didn't give a damn about canon.)
[snipped]Of course, I totally agree with Suz re: not yielding to general fan belief if it doesn't suit you. I am a huge proponent of writing whatever it is you damned please, canon be damned, anyone's opinion be damned -- as long as you can make us BELIEVE it, and that is the major critical issue here. 
Tracking the Elusive Fanon
Just where did that bit of fanon originate? Figuring it out can be difficult, as well as a lot of fun!
Fan's creativity knows no bounds, but what becomes fanon is elusive. A fan in 1999 mentioned several of her speculations that had been picked up by other fans. "And in Star Wars fanfic, I'm personally responsible for the convention that Palpatine's first name is "Bob", and for Masters cutting and braiding their Padawan's hair and weaving their own strands into the braid ("Scissors", 09 Jun 1999). It's interesting to watch these things spring up. I bet most of the people who use the cutting/braiding bit of fanon now, only a few months later, have no idea where it came from." 
- Canon/fanon; archive link by Jane Davitt (2004)
- More Than You Ever Wanted to Know About Canon and Fanon, Archived version by Melusina (2004)
- Dean Winchester and Commander Shepard Walk Into A Bar: Why Fanon Matters - Uncanny Magazine, Archived version
- Avon's Allergies dated July 30, 1993. In the discussion, fans traced the origins of Avon's "allergies" (Avon was allergic to interrogation drugs) back to the story "Between Black and White" by Ann O'Neil, Pat Thomas and Catherine Knowles. It appeared in the fanzine Best of Spacefall, put out by Horizon.
- Sandy Hereld's Feb 22, 1993 post to Virgule-L, quoted with permission.
- "A reference to "fan canon" on another list and all the zine reading I've been doing lately reminded me that when I first started reading fan fiction I noticed that many authors share a lot of ideas and assumptions....." Sue Clerc's post to Lysator dated March 9, 1993.
- Subject: The use of the word "canon" dated July 26, 1993.
- Subject: The use of the word "canon" dated July 29, 1993.
- Source: Morgan Dawn's personal notes accessed September 19, 2013.
- Subject: Friscon Report (Long) by Jungle Kitty on Oct 26, 1998.
- Emily Salzfass on April 1, 1998, at alt.startrek.creative.erotica.moderated.
- See you can't take something OFF the internet, that's like trying to take pee out of a swimming pool dated March 19, 2006.
- Transcript of the episode "The Valkyrie" mentioning the Seacouver Community Center appearing in that episode. Accessed 3 December 2008
- from the editorial of R & R #14
- In August 2015, Leslye Lilker added to Fanlore: "Apologies to Collette Mak. 40 years ago I was a real jerk. I should have been honored that Collette thought enough of the term to use it. Besides, I didn't create it. It was given to me by Trinette Kern. The place is Val'jn'jt these days, and if you use it -- well, thanks. -- added to Fanlore by L. Lilker 8/2015.
- Subject: Re: [ASCEML Re: Newbie Questions] post to alt.startrek.creative.erotica.moderated dated Nov 8, 1999.
- Dorothy Marley. The Seven Deadly Fanons of Characterization: a Spotter's Guide. Fanfic Symposium, posted 14 October 2003. Accessed 3 December 2008.
- Victoria P. like a garment made to measure (on canon and fanon). Posted 8 March 2004. Accessed 3 December 2008.
- Vee_fic untitled entry. Posted 15 May 2007. Accessed 24 December 2008.
- See Jundland, Too issue #1. See also Remix.
- from her LoC in Pegasus #4 v.1, commenting on a story in the previous issue
- Canon? or Fanon? Accessed December 3, 2008
- Fabu. More than you ever wanted to know about canon and fanon. Posted 20 February 2004. Accessed 3 December 2008.
- Flamingo: comments at a January 2000 post at The Pits, now offline, quoted on Fanlore with Flamingo's permission
- Laura Jacquez Valentine, alt.startrek.creative.erotica.moderated, November 1999