The Stacks project

Situation 52.16.1. Here $A$ is a Noetherian ring and $I \subset \mathfrak a \subset A$ are ideals. We set $X = \mathop{\mathrm{Spec}}(A)$, $Y = V(I) = \mathop{\mathrm{Spec}}(A/I)$, and $Z = V(\mathfrak a) = \mathop{\mathrm{Spec}}(A/\mathfrak a)$. Furthermore $U = X \setminus Z$.