Fanon

Fanon is a portmanteau word comprised out of the words "fan" and "canon". It refers to so-called "facts" that a significant number of fans think are true, but for which there is no evidence in any official material. A piece of fanon usually becomes 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 via retcon or other means if a writer decides to establish the fans' interpretation in official fiction. Sometimes, this can also happen because an official writer mistakes fanon for established canon.

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 Predacon ship being the Darkside or Darksyde.
 * The Armada beast redecoes being Unicron's "horsemen".
 * "Herald of Unicron"
 * Subspace as an interdimensional storage medium
 * Hot Shot is crazy and craves jam
 * U-Haul Robot from the cartoon is Sentinel Prime.

Fanon terms or ideas that never became canon:
 * Dion having been rebuilt into somebody "important"
 * Metrotitan being a "zombie" Metroplex, or cloned from a severed leg.

Fanon and this wiki
generally tries to avoid including fanon, until it becomes so widespread that excluding it is quixotic. Even then, however, fanon should be clearly identified as such. The best way to reference fanon in articles without interrupting the coverage of actual canon is to confine it to trivia notes sections (or, if really necessary, clearly marked notes within a paragraph.)