Definition:Proportion/Joint

Definition
Two real variables $x$ and $y$ are jointly proportional to a third real variable $z$ iff the product of $x$ and $y$ is a constant multiple of $z$:
 * $\forall x, y \in \R: x y \propto z \iff \exists k \in \R, k \ne 0: x y = k z$

Also known as
The term proportion is more usually known nowadays by the word proportionality.