The constant of variation can also be referred to as the constant of proportionality

Well lets look at the definitions and formulas of both to compare.

The formula of Constant of Variation: 
Since k is constant (the same for every point), we can find k when given any point by dividing the y-coordinate by the x-coordinate. For example, if y varies directly as x , and y = 6 when x = 2 , the constant of variation is k = = 3 . Thus, the equation describing this direct variation is y = 3x .

The formula of Constant of Proportionality:
The constant is called the coefficient of proportionality orproportionality constant. If one variable is always the product of the other and a constant, the two are said to be directly proportional. x and y are directly proportional if the ratio yx is constant.

Compare them and you'll see the answer,
I hope i helped, and tag me if you need anything :)

Thank You,
Darian