Headcanon
(also written as “head canon,” but it’s really one word.)
An idea, belief, or aspect of a story that is not mentioned in the media itself, but is accepted by either the media consumer themselves or the fandom in general. If it is confirmed by the creator of the story, it becomes canon.
In Homestuck , a popular webcomic , many suspect the headcanon that the trolls are carnivores, because of their natural homicidal tendencies and sharp teeth. It’s never explicitly stated in the comic that this is true, but it’d be kind of dumb to assume they’re a race known for salads and ice cream sundaes.