Grothendieck Site
A site specifies which families of maps count as coverings, allowing sheaf methods to work beyond open subsets of a space. Begin with the covering axioms and the Zariski example before moving to sites of schemes.
Before opening the references
Where to read
Start here: The Stacks Project — Sites
The Stacks Project Authors · Online reference · English · Free full text
Section 7.6, Definition 7.6.2 and Example 7.6.4
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Algebraic Geometry in the library · Preparation and skip guidance