The viscosity characterizes the flow properties of a liquid or a gas. Viscosity can also be described as internal friction of the fluid. Higher viscosity means low fluidity and low viscosity high fluidity. The viscosity depends on the molecular structure of the fluid components. Depending on the molecular structure varying attractive forces between the molecules of the fluid occur. High attractive forces result in high viscosity, low attractive forces in low viscosity of the fluid. In a simplyfied model, a fluid can be considered as a stack of molecular layers. In the state of flow the molecule layers are sliding along each other. To overcome the resistance between the molecules against displacement caused by the attractive forces, a certain power is required. The dynamic viscosity is defined as constant of proportionality in the correlation between required force and induced velocity gradient (shear rate).

Dynamic viscosity = shear stress / shear rate. The dynamic viscosity is measured in Pascal second (Pa • s = 1 Ns/m²).

Under the influence of gravity the kinematic viscosity is the ratio of dynamic viscosity and density (i.e. e.g. in case of free flow measurement by means of capillary viscosimeter, see Ubbelohde-Viscosimeter).