If sufficient external force is applied the natural shape and size will be changed.

This distortion or deformation of the material is known as the ~strain~.

Every stress produces a corresponding strain, and within a certain limit (see _elastic limit_, in FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS, above) the strain is directly proportional to the stress producing it.[1] The same intensity of stress, however, does not produce the same strain in different materials or in different qualities of the same material.

No strain would be produced in a perfectly rigid body, but such is not known to exist.
[Footnote 1: This is in accordance with the discovery made in 1678 by Robert Hooke, and is known as _Hooke's law_.] Stress is measured in pounds (or other unit of weight or force).
A ~unit stress~ is the stress on a unit of the sectional { P } area.

{ Unit stress = -- - } For instance, if a load (P) of one { A } hundred pounds is uniformly supported by a vertical post with a cross-sectional area (A) of ten square inches, the unit compressive stress is ten pounds per square inch.
Strain is measured in inches (or other linear unit).