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!