Definition:Continuous Extension

Definition
Let $T_1 = \left({S_1, \tau_1}\right)$ and $T_2 = \left({S_2, \tau_2}\right)$ be topological spaces.

Let $A, B \subseteq S_1$ be subsets of $S_1$ such that $A \subseteq B$.

Let $f: A \to S_2$ and $g: B \to S_2$ be continuous mappings.

Then $g$ is a continuous extension of $f$ iff:
 * $\forall s \in A: f \left({s}\right) = g \left({s}\right)$

That is, a continuous extension of $f$ is a continuous mapping on a superset which agrees with $f$ on the domain of $f$.

Simply, it is a continuous mapping which is an extension.