Let $T = (X, \mathcal{T})$ be a D1106: Topological space.
A D11: Set $E \subseteq X$ is topologically dense in $T$ if and only if \begin{equation} \forall \, U \in \mathcal{T} \left( U \neq \emptyset \quad \implies \quad U \cap E \neq \emptyset \right) \end{equation}