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Viscosity is a property of the material which represents the resistance to continuous deformation or flow. Fluids are typically described in terms of shear viscosity, defined as the ratio between shear stress and shear rate. Unlike elasticity, the stress is related to the rate of deformation and not to the deformation. In a fluid, if an external stress is exerted, deformation occurs and the material flows indefinitely until the stress is removed. A Newtonian fluid (e.g. water) is characterized by constant viscosity for any shear rate. Most polymer solutions and melts are non-Newtonian fluids, whose viscosity decreases as the shear rate increases (shear thinning behavior). Temperature conditions affect significantly the level of viscosity.