1. 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.
    • 1611, The Holy Bible, […] (King James Version), imprinted at London: By Robert Barker, […], OCLC 964384981 ↗, Job 38:37–38 ↗:
      Who can number the cloudes in wiſedom? or who can ſtay the bottles of heauen, / When the duſt groweeh{{sic
  2. An instance of this quality; hardship.
  3. (inorganic chemistry) 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. (physics) The penetrating ability of electromagnetic radiation, such as x-rays; generally, the shorter the wavelength, the harder and more penetrating the radiation.
  6. The measure of resistance to damage of a facility, equipment, installation, or telecommunications infrastructure when subjected to attack.

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