What is Proportionality constant ?

Proportionality constant:

Two varying quantities are said to be in a relation of proportionality when either their ratio or their product yields a constant.

The value of this constant is called the coefficient of proportionality or proportionality constant.

If the ratio $\frac{y}{x}$ of two variables ($x$ and $y$) is equal to a constant ($k =\frac{y}{x}$), then the variable in the numerator of the ratio ($y$) can be the product of the other variable and the constant ($y = k ⋅ x$).

In this case, $y$ is said to be directly proportional to $x$ with proportionality constant $k$. 

If the product of two variables ($x ⋅ y$) is equal to a constant ($k = x ⋅ y$), then the two are said to be inversely proportional to each other with the proportionality constant $k$.