 Shear stress

Shear stress is the tangential component of traction on a surface. For leading unidirectional flow $u(y)$ in a Newtonian fluid, $\tau =\mu du/dy$.

Shear stress is a measure of the intensity of internal forces acting parallel to a surface in a material. It arises when a force is applied tangentially to an area of a material, causing the layers of the material to slide past one another.