The following equation illustrates the relation between shear rate and shear stress for a fluid with laminar flow only in the direction x:
where:
*  is the shear stress in the components x and y, i.e. the force component on the direction x per unit surface that is normal to the direction y (so it is parallel to the direction x)
*  is the viscosity, and
*  is the flow velocity gradient along the direction y, that is normal to the flow velocity .
If viscosity is constant, the fluid is Newtonian.