Let $T = (X, \mathcal{T})$ be a D1106: Topological space.
A D62: Sequence $x : \mathbb{N} \to X$ is convergent in $T$ if and only if \begin{equation} \exists \, a \in X : \forall \, U \in \mathcal{T} \left( a \in U \quad \implies \quad \exists \, N \in \mathbb{N} : \forall \, n \geq N : x_n \in U \right) \end{equation}