Newtonian fluid
A Newtonian fluid is one which has a linear relationship between stress and strain rate: the ratio of stress to strain rate represents the viscosity of the fluid. 
Figure: Stress and strain rate of a Newtonian fluid.
A Newtonian fluid is one which has a linear relationship between stress and strain rate: the ratio of stress to strain rate represents the viscosity of the fluid.
Figure: Stress and strain rate of a Newtonian fluid.