Definition 38.15.2. In Situation 38.15.1 we say a diagram (38.15.1.1) defines an impurity of $\mathcal{F}$ above $s$ if $\xi \in \text{Ass}_{X_ T/T}(\mathcal{F}_ T)$ and $\overline{\{ \xi \} } \cap X_ t = \emptyset $. We will indicate this by saying “let $(g : T \to S, t' \leadsto t, \xi )$ be an impurity of $\mathcal{F}$ above $s$”.
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