Headcanon
Etymology
A blend of head and canon, from fanfiction and media-fandom communities on LiveJournal and Tumblr in the late 2000s. Where canon is what the official work establishes, a headcanon lives only in the fan’s own head — an interpretation held as true regardless of the source.
noun · /HED-kan-uhn/
A fan’s personal, unofficial interpretation of a story that they treat as true for themselves.
What it names
It names the private layer of belief fans build on top of a work — details, backstories, and relationships the source never confirmed but which the fan holds as real. It marks the line between what the work says and what a reader has decided to believe.
Example
It’s my headcanon that the two rivals end up running a bakery together, whatever the show says.
Related idea
The opposite of canon: canon is binding on everyone, while a headcanon binds only the fan who holds it.