Headcanon

A headcanon is a personal conclusion, often based on limited evidence or information, that an individual parades around 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, often without actual confirmation of any facts forming that conclusion.