Fanon

Fanon is the set of characters and characteristics of a fandom that is created by the fans or a fan, as opposed to canon, which is created by the actual author. It can mean ideas from a specific fanfic writer or beliefs held be many fans. People will sometimes write fanfic using the fanon created in someone else's fanfiction. This can be out of respect for that fanwriter or because they're too damn lazy to think up their own story, but if it isn't credited then it can be considered plagiarism. Fanon can also refer to a common misconception, such as that Thranduil is abusive or Elves are immune to the cold. They are spread around until people who don't know much about the fandom just assume that they're canon.