\begin{definition}[Definition:Retraction (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 $T_1$.
That is:
:$S_2 \subseteq S_1$
Let $f: S_1 \to S_2$ be a mapping.
Then $f$ is '''retraction''' of $T_1$ {{iff}}
:$\forall s \in S_2: \map f s = s$
\end{definition}