Definition:Retract (Topology)

Definition
Let $T_1 = \struct {S_1, \tau_1}$ be a topological space.

Let $T_2 = \struct {S_2, \tau_2}$ be a topological subspace of $T_1$.

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