\begin{definition}[Definition:Isometry (Metric Spaces)/Into]
Let $M_1 = \struct {A_1, d_1}$ and $M_2 = \struct {A_2, d_2}$ be metric spaces or pseudometric spaces.
Let $\phi: A_1 \to A_2$ be an injection such that:
:$\forall a, b \in A_1: \map {d_1} {a, b} = \map {d_2} {\map \phi a, \map \phi b}$
Then $\phi$ is called an '''isometry (from $M_1$) ''into'' $M_2$'''.
That is, an '''isometry (from $M_1$) ''into'' $M_2$''' is an isometry which is not actually a surjection, but satisfies the other conditions for being an '''isometry'''.
\end{definition}