The portion of a set with a certain characteristic. This might be expressed as a fraction or percentage of the whole.

If two quantities x and y are related by an equation y = kx, where k is a constant, then y is said to be (directly) proportional to x, which may be written y ∝ x. The constant k is the constant of proportionality. It is also said that y varies directly as x. When y is plotted against x, the graph is a straight line through the origin.

If y = k/x, then y is inversely proportional to x. This is written y ∝ 1/x, and it is said that y varies inversely as x.