- The quality of being hard.
- c. 1380s, [Geoffrey Chaucer; William Caxton, editor], The Double Sorow of Troylus to Telle Kyng Pryamus Sone of Troye [...] [Troilus and Criseyde] (in Middle English), [Westminster]: Explicit per Caxton, published 1482, OCLC 863541017 ↗; republished in [William Thynne], editor, The Workes of Geffray Chaucer Newlye Printed, […], book II, [London]: Printed by [Richard Grafton for] Iohn Reynes […], 1542, OCLC 932884868 ↗, folio clxxx, recto ↗:
- For truſteth wel, to longe ydone hardneſſe / Cauſeth diſpyte ful often for diſtreſſe
- For trust this well: Too long maintained hardness / Creates contempt from distress.
- 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.