Definition 75.23.2. Let $S$ be a scheme. Let $X \to Y \to Z$ be morphisms of algebraic spaces over $S$. Let $\mathcal{F}$ be a quasi-coherent $\mathcal{O}_ X$-module. Let $x \in |X|$ be a point and denote $z \in |Z|$ its image.

We say

*the restriction of $\mathcal{F}$ to its fibre over $z$ is flat at $x$ over the fibre of $Y$ over $z$*if the equivalent conditions of Lemma 75.23.1 are satisfied.We say

*the fibre of $X$ over $z$ is flat at $x$ over the fibre of $Y$ over $z$*if the equivalent conditions of Lemma 75.23.1 hold with $\mathcal{F} = \mathcal{O}_ X$.We say

*the fibre of $X$ over $z$ is flat over the fibre of $Y$ over $z$*if for all $x \in |X|$ lying over $z$ the fibre of $X$ over $z$ is flat at $x$ over the fibre of $Y$ over $z$

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