Lemma 29.36.14. Let $f : X \to S$ be a morphism of schemes. Let $x \in X$ be a point. Let $V \subset S$ be an affine open neighbourhood of $f(x)$. The following are equivalent
The morphism $f$ is étale at $x$.
There exist an affine open $U \subset X$ with $x \in U$ and $f(U) \subset V$ such that the induced morphism $f|_ U : U \to V$ is standard étale (see Definition 29.36.1).