The Stacks project

Definition 67.14.4. Let $S$ be a scheme. Let $X$ be an algebraic space over $S$. Let $Z, Y \subset X$ be closed subspaces corresponding to quasi-coherent ideal sheaves $\mathcal{I}, \mathcal{J} \subset \mathcal{O}_ X$. The scheme theoretic intersection of $Z$ and $Y$ is the closed subspace of $X$ cut out by $\mathcal{I} + \mathcal{J}$. Then scheme theoretic union of $Z$ and $Y$ is the closed subspace of $X$ cut out by $\mathcal{I} \cap \mathcal{J}$.