Vickers Hardness, Brinell Hardness, Hardness, Rockwell Hardness
The resistance of a metal to plastic deformation usually by indentation using a diamond or a hardened steel ball.
There are various recongnised hardness scales including Vickers (VPN), Brinell and Rockwell.
N.B. The empirical, but robust, relationship between hardness and tensile properties that applies in steels does not apply, and the steel tables must not be used for aluminium and its alloys. A less rigorous relationship has been established for some aluminium alloys but it is not widely used.