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Viscosity is a fluid characteristic due to molecular interaction by which an internal friction opposes deformation.

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.

Also see:
Newtonian Fluid
Non-Newtonian Fluids