Fanon

Fanon is a fact or ongoing situation in Fan fiction stories related to a television program, book or movie that has been used so much by fan writers that it has been more or less established as having happened in the fictional world, but it has not actually been established as having happened on the show, book or movie itself.

The word is a play on "fan canon".

For example, it is "canon" that Donna lied to Congress about her diary (The West Wing), but it has become "fanon" that she lied about her diary to protect Josh (because she had written in it about his PTSD, which-- if anyone knew about it-- could damage his career). Since we were never told WHY she lied about her diary, any reason the fan base comes up with is "fanon," even if it is a pretty darn good explanation.¹

¹ Reference: [1]

Other fanons include:

• The Star Wars Emperor Palpatine's first name, Cos
• Han Solo was formerly an Imperial Navy officer who left the service after deciding to free Chewbacca from slavery
• In the Doctor Who universe, the reason why The Doctor stole his TARDIS was to flee Gallifrey which was consumed with the Prydonian Academy Rebellion which was instigated by The Master.
• The real reason that the Doctor Who enemy, Davros, survived the betrayal by the Daleks was that he prepared defenses in his life support chair after learning about the treacherous nature of his creations from The Doctor.
• The real name of Number 6 in Patrick McGoohan's television series, The Prisoner, is John Drake, the hero of McGoohan's previous series Danger Man aka Secret Agent.
• Lt. Pavel Chekov served on the USS Enterprise in the period depicted in the first season of Star Trek before being promoted to the Bridge crew in the second season and encountered Khan Noonien Singh during that time.
• In the Super Mario games Wario & Waluigi are brothers (or at least half-brothers)