Let $(X,\mathcal{T})$ be a \reftext{def:topological-space-2026a}{topological space}, and let $A\subseteq X$. We say that $A$ is closed in $X$ if its \reftext{def:complement-subset-relative-set-2026a}{complement relative to $X$} is open, that is, if $X\setminus A\in\mathcal{T}$.