noun (n)

• the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale(noun.attribute)

  Antonym:
  softness
  

  source: wordnet30
• a quality of water that contains dissolved mineral salts that prevent soap from lathering(noun.attribute)

  Example:
  The costs of reducing hardness depend on the relative amounts of calcium and magnesium compounds that are present.
  

  source: wordnet30
• devoid of passion or feeling; hardheartedness(noun.attribute)

  Synonym:
  callosity, callousness, insensibility, unfeelingness
  

  source: wordnet30
• the quality of being difficult to do(noun.attribute)

  Example:
  He assigned a series of problems of increasing hardness.
  

  Synonym:
  ruggedness
  

  source: wordnet30
• excessive sternness(noun.attribute)

  Synonym:
  harshness, inclemency, rigor, rigorousness, rigour, rigourousness, severeness, severity, stiffness
  

  source: wordnet30
• The quality or state of being hard, literally or figuratively.(noun)

  source: webster1913