Shear Stress: Shear stress is the force per unit area that causes one layer of fluid to slide past another layer. It is directly related to viscosity.
Newtonian Fluid: A Newtonian fluid is a type of fluid that follows Newton's law of viscosity, where the shear stress is directly proportional to the rate of deformation.
Non-Newtonian Fluid: A non-Newtonian fluid does not follow Newton's law of viscosity and its behavior depends on factors like pressure, temperature, or applied force. Examples include ketchup and toothpaste.