hardness

Short definition from Wiktionary

Noun

1. 1
   The quality of being hard.
2. 2
   An instance of this quality; hardship.
3. 3
   The quantity of calcium carbonate dissolved in water, usually expressed in parts per million (ppm).
4. 4
   The resistance to scratching, cutting, indentation or abrasion of a metal or other solid material.
5. 5
   The penetrating ability of electromagnetic radiation, such as x-rays; generally, the shorter the wavelength, the harder and more penetrating the radiation.
6. 6
   The measure of resistance to damage of a facility, equipment, installation, or telecommunications infrastructure when subjected to attack.
7. 7
   A measure of how hard a material is