\begin{definition}[Definition:Proportion/Constant of Proportion]
Let the two real variables $x$ and $y$ be '''proportional''':
:$\forall x, y \in \R: x \propto y \iff \exists k \in \R, k \ne 0: x = k y$
The constant $k$ is known as the '''constant of proportion'''.
\end{definition}