Liquids have different properties. Besides their density or surface tension, this also includes the viscosity. Expressed in simplified terms, viscosity describes the flow characteristics of a liquid or a fluid. Viscous media feature a high viscosity, while fluid media are characterized by a low viscosity.

The viscosity is the property of a liquid or a fluid to put up a resistance to the mutual shift of two neighboring layers. Since intermolecular forces play a role here, it is also referred to as internal friction. The viscosity of a material depends on the temperature, and a distinction is made between dynamic and kinematic viscosity.

The viscosity of a medium to be transported and its dependence on the temperature must be observed in the installation planning and the selection of the matching pump. The higher the viscosity of a fluid, the higher the motor output of a pump must be, as an example.