Definition:Closure (Topology)/Definition 6

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

Let $H \subseteq S$.

$a \in S$ is in the closure of $H$ (in $T$), denoted $H^-$, :
 * for every neighborhood $N$ of $a : N \cap A \ne \O$

Also see

 * Equivalence of Definitions of Topological Closure