Definition:Closure (Topology)/Definition 5

Definition
Let $T = \left({S, \tau}\right)$ be a topological space.

Let $H \subseteq S$.

The closure of $H$ (in $T$) is the union of the set of all isolated points of $H$ and the set of all limit points of $H$:
 * $H^- := H^i \cup H'$

Also see

 * Equivalence of Definitions of Topological Closure