A property of fluids and slurries that indicates their resistance to flow, defined as the ratio of shear stress to shear rate. Viscosity can be expressed mathematically as follows:

Poise is the unit for viscosity, equivalent to dyne-sec/cm². Because one poise represents a high viscosity, 1/100 poise, or one centipoise (cp), is used for mud measurements. One centipoise equals one millipascal-second. Viscosity must have a stated or an understood shear rate in order to be meaningful. Measurement temperature also must be stated or understood.