A mathematical relationship between two quantities that vary in a constant ratio. For example, the viscosity of a Newtonian fluid is the ratio of the shear stress to shear rate over all values, $\tau \propto \gamma$. That is, the shear stress, τ, is directly proportional to the shear rate, γ. The proportionality constant is the constant that links the two quantities. In this case, it is the viscosity, μ. If the quantities are inversely proportional, then the product is a constant. For example, Boyle’s law states that the volume of a fixed mass of gas is inversely proportional to its pressure at constant temperature.