Headcanon is a slang term for someone’s personal interpretation of or belief about the details of a fictional story that aren’t part of the “official” account.In pop culture, the word canon refers to the aspects of a story or fictional “world” or “universe” that are considered to be “official”—meaning they have been confirmed within the story or in some other way (for example, an author or director might confirm something to be canon in an interview or in bonus material).
In contrast, headcanon is simply what a fan believes (or wants) to be true about a story. It can involve backstory, what happens after the story ends, or any other aspect.
Tell me your headcanon. Write that story for me. (Yes you can ship your characters)