Let $T = (X, \mathcal{T})$ be a D1106: Topological space such that

(i)	$E \subseteq X$ is a D78: Subset
(ii)	$\mathsf{int}(E)$ is the D519: Set interior of $E$ in $T$

Then $x \in \mathsf{int}(E)$ if and only if \begin{equation} \exists \, U \in \mathcal{T} : x \in U \subseteq E \end{equation}