Definition 10.149.2. Let R \to S be a formally unramified ring map.
The universal first order thickening of S over R is the surjection of R-algebras S' \to S of Lemma 10.149.1.
The conormal module of R \to S is the kernel I of the universal first order thickening S' \to S, seen as an S-module.
We often denote the conormal module C_{S/R} in this situation.
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