Stress is the internal distribution of force per unit area that balances and reacts to external loads applied to a body.

Strain is the geometrical expression of deformation caused by the action of stress on a physical body. Strain is calculated by first assuming a change between two body states: the beginning state and the final state. Then the difference in placement of two points in this body in those two states expresses the numerical value of strain. Strain therefore expresses itself as a change in size and/or shape. If strain is equal over all parts of a body, it is referred to as homogeneous strain; otherwise, it is inhomogeneous strain.