Definition 92.19.2. Let $\mathit{Sch}_{fppf}$ be a big fppf site. Let $S \to S'$ be a morphism of this site. If $p : \mathcal{X} \to (\mathit{Sch}/S)_{fppf}$ is an algebraic stack over $S$, then $\mathcal{X}$ viewed as an algebraic stack over $S'$ is the algebraic stack

$\mathcal{X} \longrightarrow (\mathit{Sch}/S')_{fppf}$

gotten by applying construction A of Lemma 92.19.1 to $\mathcal{X}$.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).