Definition:Retract (Topology)

Definition
Let $T_1 = \left({S_1, \tau_1}\right)$ be a topological space.

Let $T_2 = \left({S_2, \tau_2}\right)$ be a topological subspace of $T_1$.

Then $T_2$ is a retract of $T_1$
 * there exists a retraction $f: S_1 \to S_2$ of $T_1$.