A simplified power law relationship used to describe non-Newtonian fluids as $\tau =a{\gamma }^n.$ Depending on the value of the power index, n, the fluid can be classified as being pseudoplastic (n < 1), Newtonian (n = 1), or dilatant (n > 1). It is named after German chemist Friedrich Wilhelm Ostwald (1853–1932) and British chemist Armand de Waele (1887–1966).