Definition 18.28.1. Let $\mathcal{C}$ be a category. Let $\mathcal{O}$ be a presheaf of rings.

A presheaf $\mathcal{F}$ of $\mathcal{O}$-modules is called

*flat*if the functor\[ \textit{PMod}(\mathcal{O}) \longrightarrow \textit{PMod}(\mathcal{O}), \quad \mathcal{G} \mapsto \mathcal{G} \otimes _{p, \mathcal{O}} \mathcal{F} \]is exact.

A map $\mathcal{O} \to \mathcal{O}'$ of presheaves of rings is called

*flat*if $\mathcal{O}'$ is flat as a presheaf of $\mathcal{O}$-modules.If $\mathcal{C}$ is a site, $\mathcal{O}$ is a sheaf of rings and $\mathcal{F}$ is a sheaf of $\mathcal{O}$-modules, then we say $\mathcal{F}$ is

*flat*if the functor\[ \textit{Mod}(\mathcal{O}) \longrightarrow \textit{Mod}(\mathcal{O}), \quad \mathcal{G} \mapsto \mathcal{G} \otimes _\mathcal {O} \mathcal{F} \]is exact.

A map $\mathcal{O} \to \mathcal{O}'$ of sheaves of rings on a site is called

*flat*if $\mathcal{O}'$ is flat as a sheaf of $\mathcal{O}$-modules.

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