The Stacks project

Definition 76.7.2. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of algebraic spaces over $S$. The sheaf of differentials $\Omega _{X/Y}$ of $X$ over $Y$ is sheaf of differentials (Modules on Sites, Definition 18.33.10) for the morphism of ringed topoi

of Properties of Spaces, Lemma 66.21.3. The universal $Y$-derivation will be denoted $\text{d}_{X/Y} : \mathcal{O}_ X \to \Omega _{X/Y}$.