If an external stress is exerted on a fluid, the latter will flow to a degree that is determined by internal friction forces derived from internal molecular interactions and by the magnitude of the external stress applied. The measure of resistance to flow is defined as the viscosity of the fluid. The coefficient of viscosity, η, historically termed the absolute viscosity, is defined by Newton's law of viscous flow: σ ⁢   =     η ⁢   ⋅ γ ⋅ (1) σ = shear stress, defined as the applied stress that causes successive parallel layers of a material body to move in their own planes relative to each other γ ⋅ = shear rate, defined as the rate of change of shear strain with time