The Stacks project

Definition 29.35.1. Let $f : X \to S$ be a morphism of schemes.

1. We say that $f$ is unramified at $x \in X$ if there exists an affine open neighbourhood $\mathop{\mathrm{Spec}}(A) = U \subset X$ of $x$ and affine open $\mathop{\mathrm{Spec}}(R) = V \subset S$ with $f(U) \subset V$ such that the induced ring map $R \to A$ is unramified.

2. We say that $f$ is G-unramified at $x \in X$ if there exists an affine open neighbourhood $\mathop{\mathrm{Spec}}(A) = U \subset X$ of $x$ and affine open $\mathop{\mathrm{Spec}}(R) = V \subset S$ with $f(U) \subset V$ such that the induced ring map $R \to A$ is G-unramified.

3. We say that $f$ is unramified if it is unramified at every point of $X$.

4. We say that $f$ is G-unramified if it is G-unramified at every point of $X$.