Hardness is the property of a material that enables it to resist plastic deformation, usually by penetration.

The above relative hardness tests are limited in practical use and do not provide accurate numeric data.

The usual method to achieve a hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape, with a specific force applied for a specific time.

There are three principal standard test methods for expressing the relationship between hardness and the size of the impression, these being Brinell, Vickers, and Rockwell.