\begin{definition}[Definition:Rational Number Space]
Let $\Q$ be the set of rational numbers.
Let $d: \Q \times \Q \to \R$ be the Euclidean metric on $\Q$.
Let $\tau_d$ be the topology on $\Q$ induced by $d$.
Then $\struct {\Q, \tau_d}$ is the '''rational number space'''.
\end{definition}