Headcanon was originally a term similar to fanon that is used in fanfiction circles to refer to the way a person views a character’s backstory. It originates from the term “canon” meaning a collection of formally approved materials and facts within a fictional universe established by the original creator or subsequent creators who have been given authority to add to that universe. The term was firest used in literature to refer to Sherlock Holmes stories written by Arthur Conan Doyle, and it is currently used to establish a continuity between characters and events within fiction that encompasses multiple books, movies, games, or other media.
Headcanon takes this concept away from the approved universe, but still applies continuity to the creative process, so that a writer (artist, director, etc.) who has not been sanctioned by the original creator of the materials can adhere to a character’s motivations and fictional life, thus maintaining the audience’s suspension of disbelief.
Basically, headcanon means that a story follows the rules already officially established, even if the story’s creator is just a fan.
The term has come into heavier use with the popularity of Game of Thrones, Star Wars, Star Trek, and other media with a significant number of characters and events that need to be tracked so that plot holes don’t inadvertently develop. Even official creators use the term. Often fans will ask creators and writers questions, such as if a certain character is actually pretending to be evil, or if a particular location was actually known by another name in a previous part of the story. If these questions can’t be answered “in canon” by referring to existing material, a creator might say that the answer is yes or no according to her headcanon, meaning that she operates according to rules she has set in her head, even though they have not been officially established at that point.
Nerds take an official creator’s headcanon seriously, because it can provide clues to where a story is going, or where it may have come from, allowing us to adjust our own headcanon to fit. We tend to speculate about the meaning of even minute details in our favorite books and movies, and while referring to headcanon doesn’t carry as much weight as referring to an actual passage from a book or scene in a movie, it’s still more reliable than sheer speculation.
It’s likely that you have your own chronology of events and imagined character backstories for books, movies, or games, and even if you weren’t aware it existed until today, now you have a name for it.
