\begin{definition}[Definition:Closure (Topology)/Definition 1]
Let $T = \struct {S, \tau}$ be a topological space.
Let $H \subseteq S$.
The '''closure of $H$ (in $T$)''' is defined as:
:$H^- := H \cup H'$
where $H'$ is the derived set of $H$.
That is, $H^-$ is the union of $H$ and its limit points.
\end{definition}