When a force is applied to a body it deforms in some way. We are already familiar with two of these deformations, both of which are classed as volumetric strains, i.e. a uniform force is applied to every surface of a body, such as pressure, which causes the volume to decrease, compression, or increase, dilatation.
If a non-uniform force is applied, e.g. to just one side of a body, then we alter the shape, but not the volume, of the body. This is called a shearing strain. For example, a fluid can be between two parallel plates, if the lower plate is held fixed while the upper plate is moved parallel to the lower plate the body will then be sheared.
The velocity gradient response, $\dot{\gamma}=\partial u/\partial y$, caused by this shear stress, $\tau$ is due to the fluid viscosity. The general form of the equation for simple fluids can be given by,
$$\tau = \tau_0 + \mu_0\dot{\gamma}^n$$
The graph below shows how varying these parameters varies the viscosity.