Newtonian fluids, at laminar flow, create shear tension and pressure super-imposed normal tensions which are proportional to the deformation velocity, the proportionality factor being the absolute viscosity.
The kinematic viscosity is defined as:
The viscosity depends on temperature and pressure, whereby pressure dependency of fluids is of negligibly small characteristics.
Viscosity of non-Newtonian fluids can moreover be time-related (thixotropic or rheopexic flow characteristics); its characteristics then becoming undefined.
The viscosity of a medium has an influence on both the system curve and the pump performance curve. For centrifugal pumps, the pump curves are converted in practice at a kinematic viscosity of more than 10 mm²/s.