In
fandom, a body of ideas about characters, events, etc., that are not part of
canon, but are popular among fans. These may be theories widely accepted in fan discussions, or conjectural events and elements of characterization frequently used in
fanfic. Fanon elements often arise from a well-expressed theory, or from a particularly effective fanfic, and become widely accepted and used because they address a discrepancy in canon, make for good drama in fiction, or are simply aesthetically pleasing ideas.
Fanon is almost always at least compatible with, if not strongly implied by, canon, though it is seldom self-consistent, and ranges from trivialities to entire histories for certain characters. Some arise from facts about writers, actors, etc. involved in the making of the source canon, the preferences or politics of the creators being bestowed on their characters. Childhood traumas, family situations, secret passions, favorite foods, and toiletry preferences are all popular subjects of fanon.
The word fanon is frequently used in a derogatory sense. Persons who join the fandom later, or are not well-acquainted with the canon, frequently mistake fanon for canon, and may embarrass themselves by "correcting" someone who expresses an alternative theory equally compatible with canon. An idea that, in the first story where it appeared, was so well written and grounded in corroborating elements from canon that it seemed like a natural outgrowth of canon, often becomes a fanon cliche because it has been used so often and so carelessly by imitators, some of whom do not realize it is not canon in the first place. Some people use the word fanon only of ideas whose popularity is owed to their appeal to the lowest common denominator.
In cases where the canon is still open, for instance while a television series is still in production, it is not uncommon for fanon to become canon, or else to become irreconcilable with canon, and these definitions are always vulnerable to the violent disagreements in most fandoms over what constitutes canon.
Examples of fanon include Dana Scully's strawberry-scented shampoo, the widespread use of lubricative and prophylactic spells among Hogwarts students, and Rupert Giles' one-time relationship with Ethan Rayne.