# Definition:Boundary Point (Complex Analysis)

Let $S \subseteq \C$ be a subset of the complex plane.
Let $z_0 \in \C$.
$z_0$ is a boundary point of $S$ if and only if every $\epsilon$-neighborhood $\map {N_\epsilon} {z_0}$ of $z_0$ contains points of $\C$ in $S$ and also points of $\C$ which are not in $S$.