An interpretation of a piece of media that is not shown or confirmed in the original but is believed to be true simply for preference.
May also include direct deviations from the canon.
In a perfect world, instead of Philip, Grog would have recited Percy's full name perfectly as his 'fancy name' and, after a moment of stunned silence, Percy would have added, 'You can just call him Percy.'"
In my head-canon, Mario in the original Donkey Kong games from the early ‘80s is actually the father of Mario and Luigi. That Pauline is probably a different Pauline. Maybe it’s modern Pauline’s mother. The second Mario in Donkey Kong Jr. is Mario and Luigi’s random uncle.
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!
(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.