What does proportional and inversely proportional mean?

Two measures are proportional if both are zero at the same time, and increases of the same amount one are accompanied by increase of equal amounts in the other. Algebraically, Y = cX where c is some fixed number, called the constant of proportionality.

Two variables W and Z are inversely proportional if W*Z = k for some fixed number - also called the constant of proportionality. The relationship may also be written as W = k/Z.