nLab
fpqc topology

Idea

The fpqc topology is a Grothendieck topology on the category of (commutative) affine schemes. It is one of the main Grothendieck topologies used in algebraic geometry.

Definition

Let $Aff$ be the category opposite to the category of commutative algebras. A family of maps $\{f_i : U_i\to X\}_{i\in I}$ in that category is a cover in the fpqc topology if the union of images cover $X$ as a topological space and all morphisms $f_i$ are faithfully flat and quasicompact. The French for this is fidèlement plat et quasicompact (fpqc).

Related concepts

fpqc-site $\to$ fppf-site $\to$ syntomic site $\to$ étale site $\to$ Nisnevich site $\to$ Zariski site

category: algebraic geometry