hardness

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.