Definition 65.16.2. Let $S$ be a scheme. A morphism $f : X \to Y$ between algebraic spaces over $S$ is called étale if and only if for every étale morphism $\varphi : U \to X$ where $U$ is a scheme, the composition $f \circ \varphi$ is étale also.

Comment #2317 by Nithi Rungtanapirom on

In the last clause, the composition $\varphi\circ f$ should read $f\circ\varphi$.

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