Canon

Fluff supported by the creators. It should be noted that canon =/= cannon, no matter how frequently it is used as such.

Canon itself can be sometimes unclear or subject to change, especialy in works which have more than one author (Doctor Who being a notorious example) and works which have gone through more than one version. The latter can be often seen in changes that occur between editions of a roleplaying game; for example in 3rd edition D&D the transformation of a Drow into the monstrous Drider was a punishment, in 4th edition it is a blessing the godess bestows on the best of the society.