Canon is the body of material treated as the official, factual version of a story's world and events, as opposed to material invented for an adaptation, a spin-off, or a fan work. In anime fandom the anchor is usually the original work: if it is in the manga, it is canon.
Every long-running franchise eventually forces the question, because adaptations multiply. A single story may exist as a manga, two different anime, a film series, and a set of databooks, and they will not all agree. Canon is the tool fans use to decide which version of events a discussion is standing on: the facts that count, from the source that counts.
For adapted works the usual anchor is the original. Manga canon means the story as its author published it; material the anime invented is outside that canon however enjoyable it is. A grey zone of author-approved additions sits between, and fandoms argue its edges series by series, which is exactly why this site does not leave canon to vibes.
Every anime subject here carries a canon sheet stating its precedence: which continuity is the unmarked default, which variants must be labelled in a quiz's title, and which material is excluded. The quiz schema requires a canon declaration on every quiz, printed from the same sheet. Canon, here, is metadata, not manners.
In use"Asked whether an anime-only villain is canon, the precise answer is: not to the manga, and the quiz must say which it means."