Fanon

Fanon is a so-called "fact" that a fan thinks is true, but for which there is no evidence in any official material. A piece of fanon may be a belief held by just a single fan, or a belief which is widespread due to having been discussed and picked up by other fans. Some fanon is based on misinterpretations of canon (especially when translations from Japanese material are involved) and some is simply stuff fans made up. While fanon originates with fans rather than the official canon of the Transformers multiverse, it sometimes becomes incorporated into the canon if the writers use a retcon or otherwise establish the fans' interpretation in official fiction.

Misuse of the term has broadened the word's definition (among some) to include fan-characters and fan-fiction. (See "Transfanon Wiki" link below.) The difference is that fan-fiction is an extension built upon canon, while fanon is an interpretation or explanation of existing canon.

Examples

Fanon terms or ideas that later became canon:

"Gestalt" as a term for combiners
The name of the BW Predacon's ship being the Darkside or Darksyde.
The Armada beast redecoes being Unicron's "horsemen".
"Herald of Unicron" (though only used once)

Fanon terms or ideas that never became canon:

Dion having been rebuilt into somebody "important"
Metrotitan is a zombie

Fanon and this wiki

MediaWiki generally tries to avoid including fanon, until it becomes so widespread that excluding it is quixotic. Even then, however, fanon should be clearly marked as such. (E.g., Cyclonus's Armada.)