The Stacks project

Definition 34.7.5. Let $T$ be an affine scheme. A standard fppf covering of $T$ is a family $\{ f_ j : U_ j \to T\} _{j = 1, \ldots , m}$ with each $U_ j$ is affine, flat and of finite presentation over $T$ and $T = \bigcup f_ j(U_ j)$.