Definition of "hardness"

The quality of being hard.  quotations  examples

An instance of this quality; hardship.  examples

(inorganic chemistry) The quantity of calcium carbonate dissolved in water, usually expressed in parts per million (ppm).  examples

The resistance to scratching, cutting, indentation or abrasion of a metal or other solid material.  examples

(physics) The penetrating ability of electromagnetic radiation, such as x-rays; generally, the shorter the wavelength, the harder and more penetrating the radiation.  examples

The measure of resistance to damage of a facility, equipment, installation, or telecommunications infrastructure when subjected to attack.  examples

(countable, engineering) A measure of how hard a material is  examples