A shearing force is applied to the top of the rectangle while the bottom is held in place. The resulting shear stress, \tau\,, deforms the rectangle into a parallelogram. The area involved would be the top of the parallelogram.
A shear stress, denoted τ (Greek: tau), is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section. Normal stress, on the other hand, arises from the force vector component perpendicular to the material cross section on which it acts.