It truly baffles me that most people in this fandom don’t know the difference between headcanon, fanon and canon.
Headcanon: 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.
Fanon: A term used in fanfiction to describe commonly accepted ideas among authors even if they are not actually expressed in the canon work.
Canon: Another word for official. Used quite often in fan fiction to differentiate between the official storyline in which the fan fiction is based on.