## 111.8 Definition of algebraic stacks

An algebraic stack is a stack in groupoids over the category of schemes with the fppf topology that has a diagonal representable by algebraic spaces and is the target of a surjective smooth morphism from a scheme. See Algebraic Stacks, Section 92.12. A “Deligne-Mumford stack” is an algebraic stack for which there exists a scheme and a surjective étale morphism from that scheme to it as in the paper [DM] of Deligne and Mumford, see Algebraic Stacks, Definition 92.12.2. We will reserve the term “Artin stack” for a stack such as in the papers by Artin, see [ArtinI], [ArtinII], and [ArtinVersal]. A possible definition is that an Artin stack is an algebraic stack $\mathcal{X}$ over a locally Noetherian scheme $S$ such that $\mathcal{X} \to S$ is locally of finite type^{1}.

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