Fanon is a fact or ongoing situation in fan fiction stories that has been used so much by fan writers or among the fandom 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 in the books themselves. Fanon is a portmanteau word of fan and canon.
Things which are considered fanon are things that are not strictly canon, but do not contradict it and are widely accepted by most fans. The existence of the Sons of Discord offshoot of the Daughters of Cacophony, for example, is fanon.
The White Wolf Wiki is a Wiki for canon elements of the various White Wolf settings, and as such, we do not fully support fanon.
By "fully," we mean that we do support it to a limited extent. For example, the Chitra Bhanu (a psi order from Trinity) were never fully detailed in an official book; thus we have no problem mentioning the material devised for the fan-created India Underground supplement. However, that information will clearly be marked as being non-canon, and none of it will lead to additional articles here on the Wiki unless those articles themselves also contain canon elements. For example, a section describing Thallain versions of the sidhe will not link to a separate article, although a mention of High King David will certainly link to his article, as he is a canon character.
See also: canon
The WOD Wiki, a Wiki (supporting both the old and new WODs) which is intended for fan-based creations.