Quotient Stack
A quotient stack records the orbit data of a group action together with the isomorphisms between representatives. Follow the action groupoid into its stackification and track the natural map from the original space to the quotient.
Before opening the references
Scheme · Group Action · groupoid · Sheaf
Where to read
Start here: The Stacks Project — Quotient stacks
The Stacks Project Authors · Online reference · English · Free full text
Section 78.20, Definition 78.20.1 and Lemma 78.20.2
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Algebraic Geometry in the library · Preparation and skip guidance