noun

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