Shear stress

component of stress coplanar with a material cross section

Shear stress is a stress state where the stress is parallel to the surface of the material, as opposed to normal stress when the stress is perpendicular to the surface.

A road destroyed by shear.

Shear stress is relevant to the motion of fluids upon surfaces, which result in the generation of shear stress. Also constructions in soil can fail due to shear; e.g., the weight of an earth-filled dam may cause the subsoil to collapse, like a small landslide.