The Stacks project

Definition 94.16.4. Let $S$ be a scheme. Let $B$ be an algebraic space over $S$. Let $(U, R, s, t, c)$ be a groupoid in algebraic spaces over $B$. We say $(U, R, s, t, c)$ is a smooth groupoid¹ if $s, t : R \to U$ are smooth morphisms of algebraic spaces.