Definition:T4 Space/Definition 1

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

$T = \left({S, \tau}\right)$ is a $T_4$ space iff:


 * $\forall A, B \in \complement \left({\tau}\right), A \cap B = \varnothing: \exists U, V \in \tau: A \subseteq U, B \subseteq V, U \cap V = \varnothing$

That is, for any two disjoint closed sets $A, B \subseteq S$ there exist disjoint open sets $U, V \in \tau$ containing $A$ and $B$ respectively.

That is:
 * $T = \left({S, \tau}\right)$ is $T_4$ when any two disjoint closed subsets of $S$ are separated by neighborhoods.

Also see

 * Equivalence of Definitions of $T_4$ Space