When a force is applied on a body, it undergoes deformation. In order to restore the body to its original shape and/or size, an opposite or restoring force is generated within the body. This restoring force is equal to the magnitude of the applied force, but acts in the opposite direction. The amount of this restoring force developed per unit area of the body is called stress. Stress is a tensor quantity and has the SI unit pascal. Stress can be separated into four broad categories depending upon the direction of the forces acting on the body and the type of deformation the body undergoes. These include tensile stress, compressive stress, volumetric stress, and shear stress.

When the force applied to a body results in an increase or decrease in the body length, the resulting stress is termed as tensile or compressive stress, respectively. If the applied force acts on all the sides or dimensions of a body and results in a volume change, it is known as volumetric stress. If the direction of the applied force is parallel to the cross-sectional area and results in a change in the shape of a body, it is termed shear stress.

This text is adapted from Openstax, University Physics Volume 1, Section 12.3: Stress, Strain, and Elastic Modulus.