Two of the most important words in fandom are 'canon' and 'fanon,' and the line between them explains a lot about why fanfiction reads the way it does. One is the official source material; the other is the shared imagination of the fandom built on top of it.
Knowing the difference helps you understand why a character is written a certain way 'everywhere,' even when the original never said so.
Canon: the official source
Canon is everything established by the original work — the book, show, film, or game. If it happened on screen or on the page, it's canon. 'Canon compliant' fic stays consistent with it; 'canon divergence' deliberately changes it from a certain point.
Creators sometimes add canon outside the main work (interviews, supplementary material), which fandom debates endlessly about whether to count.
Fanon: what fandom invented
Fanon is the body of ideas the fandom collectively adopted that aren't actually in canon — a character's favorite coffee order, a backstory detail, a personality quirk — that becomes so widespread it feels official. It spreads when enough fics repeat it that new readers absorb it as 'true.'
Fanon is part of what makes a fandom feel like a shared world. It's also why two fandoms can read the same canon character very differently.
Frequently Asked Questions
What does canon mean in fanfiction?
The events and details established by the original source material. 'Canon compliant' fic doesn't contradict it; an AU departs from it.
What is fanon?
Ideas widely accepted within a fandom that never actually appeared in canon — invented details that became popular enough to feel official.
What's a headcanon?
A personal belief about a character or story a fan holds that isn't canon. When a headcanon spreads widely across a fandom, it becomes fanon.