Viscosity is a measure of the resistance of a fluid to deform under shear stress. It is commonly perceived as flow behaviour or resistance to pouring. Viscosity describes a fluids internal resistance to flow and may be thought of as a measure of fluid friction.

For certain liquids viscosity is a material constant that only depends on temperature and pressure. This group of materials is termed Newtonian liquids.

In practice, time-dependent viscosity is called thixotropy. If a liquid is sheared at a constant velocity gradient, viscosity will slowly decrease. As soon as the shear forces are removed, viscosity will recover and return to the initial value.