In mild or medium-carbon steel, the stress at which a marked increase in deformation occurs without an increase in load; also called proportional limit. In other steels and in nonferrous metals this phenomenon is not observed. Refer also to yield strength.

The stress at which a material exhibits a specified permanent plastic yield or set; a limiting deviation from proportionality of stress to strain. An offset of 0.2% is used for many metals, such as aluminium-based and magnesium-based alloys, while 0.5% total elongation under load is frequently used for copper alloys. Also called proof stress.