Tor measures the deviation of flatness.
Lemma 20.26.16. Let $(X, \mathcal{O}_ X)$ be a ringed space. Let $\mathcal{F}$ be an $\mathcal{O}_ X$-module. The following are equivalent
$\mathcal{F}$ is a flat $\mathcal{O}_ X$-module, and
$\text{Tor}_1^{\mathcal{O}_ X}(\mathcal{F}, \mathcal{G}) = 0$ for every $\mathcal{O}_ X$-module $\mathcal{G}$.
Proof. If $\mathcal{F}$ is flat, then $\mathcal{F} \otimes _{\mathcal{O}_ X} -$ is an exact functor and the satellites vanish. Conversely assume (2) holds. Then if $\mathcal{G} \to \mathcal{H}$ is injective with cokernel $\mathcal{Q}$, the long exact sequence of $\text{Tor}$ shows that the kernel of $\mathcal{F} \otimes _{\mathcal{O}_ X} \mathcal{G} \to \mathcal{F} \otimes _{\mathcal{O}_ X} \mathcal{H}$ is a quotient of $\text{Tor}_1^{\mathcal{O}_ X}(\mathcal{F}, \mathcal{Q})$ which is zero by assumption. Hence $\mathcal{F}$ is flat. $\square$
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Comment #2593 by Rogier Brussee on