Let $T = (X, \mathcal{T})$ be a D1106: Topological space.
A D2218: Set element $x \in X$ is an isolated point of $E \subseteq X$ in $T$ if and only if
\begin{equation}
\exists \, U \in \mathcal{T} : U \cap E = \{ x \}
\end{equation}
▶ | Set of isolated points |