Definition:Closure (Topology)/Definition 6

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

Let $H \subseteq S$.

The closure of $H$ (in $T$), denoted $H^-$, is defined by:
 * $\forall a \in S: a \in H^- \iff$ for every neighborhood $N$ of $a : N \cap H \ne \O$

Also see

 * Equivalence of Definitions of Topological Closure