Hardness

1. Hardness is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion.
2. Hardness is a measure of how resistant solid matter is to various kinds of permanent shape change when a force is applied.

Hardness is the ability of a material to resist deformation, which is determined by a standard test where the surface resistance to indentation is measured. The most commonly used hardness tests are defined by the shape or type of indent, the size, and the amount of load applied. The hardness numbers referenced constitute a nondimensioned, arbitrary scale, with increasing numbers representing harder surfaces. The two most referenced hardness test methods are Brinell hardness and Rockwell hardness, each one having a dedicated test machine with its own unique hardness scales. Hardness correlates approximately with ultimate tensile strength in metals.

Sources:

Vocabulary.com https://www.vocabulary.com/dictionary/

Словник Webster's Dictionary https://www.merriam-webster.com/

Smith Peter Metallic Materials for Piping Components // The Fundamentals of Piping Design, 2007. https://www.sciencedirect.com/science/article/pii/B9781933762043500126 p.116