\begin{definition}[Definition:Retract (Topology)]
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 $T_1$ {{iff}}
:there exists a continuous retraction $f: S_1 \to S_2$ of $T_1$.
\end{definition}