Definition:Proportion/Constant of Proportion

Definition
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.

Also known as
The constant of proportion is also known nowadays by the less elegant and more cumbersome constant of proportionality.