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 -ssume they’re a race known for salads and ice cream sundaes.
used by followers of various media of entertainment, such as television shows, movies, books, etc. to note a particular belief which has not been used in the universe of whatever program or story they follow, but seems to make sense to that particular individual, and as such is adopted as a sort of “personal canon”. headcanon may be upgraded to canon if it is incorporated into the program or story’s universe.
in my headcanon, john and mary bicker like siblings when they’re around each other… they just seem like they’re brother and sister to me!
an idea or opinion about a fictional series (book, tv, comic, or otherwise) that is true in one’s head, but has not become a canon fact.
in my head canon, jade and dave from homestuck are totally in love with each other.
definitely in no way related to hetalia, it refers to what a character from the webcomic homestuck sounds like in their heads
oh gog octopimp’s voices have so much headcanon in them
used by many hetalia f-gs to describe something that hasn’t yet happened in original canon but has happened in their head.

i.e. their own personal canon.
my headcanon says that momma america was native american actually