is: an empirical law that describes the behaviour of some fluids under a limited range of conditions. (Such fluids are called Newtonian fluids.) [P7.6]
states: that when a shear stress is applied to a fluid, the velocity gradient that it produces in the direction perpendicular to the stress is proportional to the applied shear stress, and the resulting velocity decreases with distance from the plane over which the shear stress is applied, so
where 
is a constant, known as the coefficient of viscosity, that is characteristic of the (state of the) fluid. [P7.6]
Copyright 1997, The Open University