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- 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.
quality of being hard