A fluid whose viscosity is not constant at all shear rates and does not behave like a Newtonian fluid. Most successful drilling fluids are non-Newtonian. Within that group are several general types and rheological mathematical models to describe them. Pseudoplastic is a general type of shear-thinning, non-Newtonian behavior that is desirable for drilling fluids. Bingham plastic and power-law models describe a psuedoplastic behavior using only two measurements (two parameters). The Herschel-Bulkley model is a three-parameter rheological model.