This is one in which the viscosity does not remain constant when shear stress is applied, meaning the viscosity is dependent on the shear stress. These will have a yield stress point or a measurable amount of stress the system needs before it can start flowing.Shear stress is the force required to make the layers of fluids move. The yield stress point is the point we need to reach so the fluids start moving. So a non-Newtonian fluid is one in which the viscosity changes when the force required to make the fluids move is reached. What happens at that point is what defines a few different types of these fluids.