Source material is the original work an adaptation is made from: the manga, light novel, game, or other work that came first. In fan usage "the source" is shorthand for that original and the authority it carries over the story's facts.
In any adaptation conversation, "the source" does the directional work. It settles what came first, whose version anchors canon, and where differences originate. When fans say an anime changed something, the something is measured against the source material, and when a databook resolves an argument, it is usually by quoting the source.
The term also covers an original work's raw ingredients before publication takes a shape. A mangaka's published chapters are the source for an anime; the same author's notes and databook statements are source-adjacent, canon by declaration where a series' canon sheet says so. Keeping those layers straight is most of long-running fandom's paperwork.
This site's canon sheets are written in source language for exactly that reason. Each subject declares its source material, its precedence order, and what counts as supplement versus invention. A reader who knows the source material entry of a canon sheet knows how every quiz on that subject will treat a disputed fact. Readers of the source watch adaptations with a double vision the anime-only audience never has, which is why mixed audiences argue so productively about what a change means.
In use"The anime moved the scene indoors; in the source material it happens on the school roof."