Definition 77.2.2. In Situation 77.2.1 we say a diagram (77.2.1.1) defines an *impurity of $\mathcal{F}$ above $y$* if $\xi \in \text{Ass}_{X_ T/T}(\mathcal{F}_ T)$ and $t \not\in f_ T(\overline{\{ \xi \} })$. We will indicate this by saying “let $(g : T \to Y, t' \leadsto t, \xi )$ be an impurity of $\mathcal{F}$ above $y$”.

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