The canon of fandom.

Fanon is the collection concepts and ideas that are normally used in most fan fiction, but don't really existence in the real story's canon.

If a concept was used in the canon, it is only mention once in the entire series and is never mentioned again. Fan fiction writers will most likely instigate these concepts in their fics.

often connect to fan fiction; ideas or events that are accepted by most fans, but were not actually in the original story/book/show etc.

a term used in fanfiction to describe commonly accepted ideas among authors even if they are not actually expressed in the canon work.

Something fan-made for a fandom.

Fanon is something so widely believed in the fandom that it is considered cannon even though not actually stated in the original storyline

Something that the fans of a Fandom came up with that isn't Canon to the actual story or character. Not to be confused with Headcannon, which is a personal fanon that is usually only used by you or a small group.