An adaptation is a version of a story made in a different medium: a manga turned into an anime, a light novel into a series, a game into a film. Anime is an adaptation-driven medium, and most series adapt an existing manga, novel, or game.
Adaptation is an act of translation between media with real losses and gains. A manga panel can hold a moment for as long as a reader stares; an anime must choose a duration. A novel's interior thought becomes a look, a line, or a cutaway. The adapter's job is not to copy the source but to rebuild its effects with the new medium's tools, inside a cour grid and a budget.
That is why adaptation quality is judged on judgement, not fidelity alone. Faithful productions can be inert; loose ones can be definitive. Fandom shorthand sorts adaptations by their source relationship, and the arguments that matter are about which changes served the story and which served the schedule.
On this site, an adaptation's changes are facts like any others, filed under the right column. A quiz on an anime tests the anime; the source's version belongs to a differently labelled quiz. Keeping the columns separate is what the canon declaration on every quiz is for. The credits make the relationship official: an adaptation names its source in the copyright line, and that line is the fastest factual answer to what a series is adapted from.
In use"The anime is an adaptation of the manga: same story, rebuilt for motion, voice, and weekly episodes."