Let $T = (X, \mathcal{T})$ be a D1106: Topological space.
A D11: Set $E \subseteq X$ is a G-delta set in $T$ if and only if \begin{equation} \exists \, U_0, U_1, U_2, \dots \in \mathcal{T} : E = \bigcap_{n \in \mathbb{N}} U_n \end{equation}