What is the meaning of hardness?

noun

• The property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale.
• A quality of water that contains dissolved mineral salts that prevent soap from lathering.
  • usage: "the costs of reducing hardness depend on the relative amounts of calcium and magnesium compounds that are present"
• Devoid of passion or feeling; hardheartedness.
  • synonyms: unfeelingness, callousness, callosity, insensibility
• The quality of being difficult to do.
  • usage: "he assigned a series of problems of increasing hardness"; "the ruggedness of his exams caused half the class to fail"
  • synonyms: ruggedness
• Excessive sternness.
  • usage: "severity of character"; "the harshness of his punishment was inhuman"; "the rigors of boot camp"
  • synonyms: severity, severeness, harshness, rigor, rigour, rigorousness, rigourousness, inclemency, stiffness