Headcanon
From Halcove
A headcanon is a personal conclusion that an individual regards as fact.
Its root word is "canonical", which, in this context, refers to a collection of facts that are authentic and accurate in regards to a specific subject. The "head" refers to the personal interpretation.
Therefore, a headcanon is a personal conclusion regarding something or someone that an individual chooses to regard as factual, regardless of confirmation of any facts that form the conclusion.