hardness in WordNet English dictionary

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