In a flowing fluid a measure of strain is the velocity gradient perpendicular to the direction of flow. This is the classical idealized fluid. This constant v is called the viscosity of the fluid. In common dimensionless numbers such as the Reynolds number, it is assigned the symbol . Note that for a newtonian fluid the viscosity is constant irrespective of the shear stresses and independent of time.
Newtonian behaviour is approximated by gases and some liquids,notably water.
Here the SS - SN relationship depends on how the fluid has been sheared and on previous history. These fluids are predominantly viscous, but exhibit partial elastic recovery after deformation.