Definition:Proportion

Definition
Two real variables $x$ and $y$ are proportional iff one is a constant multiple of the other:
 * $\forall x, y \in \R: x \propto y \iff \exists k \in \R, k \ne 0: x = k y$

Inverse Proportion
Two real variables $x$ and $y$ are inversely proportional iff their product is a constant:
 * $\forall x, y \in \R: x \propto \dfrac 1 y \iff \exists k \in \R, k \ne 0: x y = k$

Joint Proportion
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 $z$:
 * $\forall x, y \in \R: x y \propto z \iff \exists k \in \R, k \ne 0: x y = k z$

Constant of Proportionality
The constant $k$ is known as the constant of proportion, or (more common nowadays, but uglier) constant of proportionality.