Hardness is a very old measure used by engineers and geologists which was first utilised to establish an order for the mechanical properties of minerals. Barba  defined already 1640:
„... hardness is such a property of precious stones that those which file can scratch are not so classed.”
This explains hardness as a measure of plastic properties which are responsible for the durability or failure of machines and tools. Consequently the scratch hardness of Mohs  (1822)
was the first hardness unit that allowed a ranking of minerals between 10 reference materials. Since that time a lot of different hardness definitions and units were introduced.
The nowadays mostly accepted hardness definition is that of Martens from 1912: Hardness is the resistance of a body against the penetration of another (harder) body.
This definition means a permanent penetration due to plastic deformation because otherwise both bodies would be left unchanged after unloading and a measure can not be derived.
All common hardness tests use only normal loading and exclude lateral forces, required for a scratch, to simplify the test conditions.
However the diversity in hardness definitions makes it still complicated to compare hardness values. The ASM Metals Handbook, Vol. 8, Mechanical Testing makes the following note:
„The definition of hardness varies depending on the experience or background of the person conduction the test or interpreting the test data.
To the metallurgist, hardness is the resistance to indentation; to the design engineer, a measure of flow stress; to the lubrication engineer, the resistance to wear;
to the mineralogist, the resistance to scratching; and to the machinist, the resistance to cutting”.
A clear description under which conditions a hardness value was obtained is therefore absolutely necessary. The different hardness standards prescribe here clear rules which are unfortunately not always kept.
 H. O. Neill, Hardness Measurement of Metals and Alloys, Chapman and Hall, London (1967) 2
 F. Mohs, Grundriß der Mineralogie, Dresden, 1822