What is a direct proportion?

Answer 1

Two quantities x and y are said to be in direct proportion if whenever the value of x increase (or decrease), then the value of y increases (or decrease) in such a way that the ratio x/y remains constant.

Answer 2

Two variables, X and Y are in direct proportion if Y = kX where k is a constant of proportion.

This requires that if one of x and y is 0, then so is the other, and that for every unit increase in x, y increase by k.