noun The quantity of calcium carbonate dissolved in water, usually expressed in parts per million (ppm).
noun The resistance to scratching, cutting, indentation or abrasion of a metal or other solid material.
noun The penetrating ability of electromagnetic radiation, such as x-rays; generally, the shorter the wavelength, the harder and more penetrating the radiation.
noun The measure of resistance to damage of a facility, equipment, installation, or telecommunications infrastructure when subjected to attack.