\begin{definition}[Definition:Euclidean Metric/Ordinary Space]
Let $\R^3$ be the real vector space representation of ordinary $3$d space.
The '''Euclidean metric''' on $\R^3$ is defined as:
:$\map {d_2} {x, y} := \sqrt {\paren {x_1 - y_1}^2 + \paren {x_2 - y_2}^2 + \paren {x_3 - y_3}^2}$
where $x = \tuple {x_1, x_2, x_3}, y = \tuple {y_1, y_2, y_3} \in \R^3$.
\end{definition}