Fanon

Fanon is a body of popular suppositions and theories accepted by fans concerning the Doctor Who Universe. It is a portmanteau of the words "fan" and "canon".

Examples
Some of this derives from fans themselves and some from non-canon sources such as The Doctor Who Role Playing Game, which, for example made the Master, the Monk and the War Chief into the same Time Lord.

Some fan theories, such as the Season 6B theory, have gotten confirmed, either onscreen or in spin-off works such as novels. Others, such as the one making the Doctor and the Master into brothers, denied.