Hardness

noun

1. The quality of being hard.
2. An instance of this quality; hardship.
3. The quantity of calcium carbonate dissolved in water, usually expressed in parts per million (ppm).
4. The resistance to scratching, cutting, indentation or abrasion of a metal or other solid material.
5. The penetrating ability of electromagnetic radiation, such as x-rays; generally, the shorter the wavelength, the harder and more penetrating the radiation.