1. hardness

the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale

2. hardness

a quality of water that contains dissolved mineral salts that prevent soap from lathering

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

3. callosity, callousness, hardness, insensibility, unfeelingness

devoid of passion or feeling; hardheartedness

• callosity
• callousness
• insensibility
• unfeelingness

4. hardness, ruggedness

the quality of being difficult to do

Example Sentences:
'he assigned a series of problems of increasing hardness'
'the ruggedness of his exams caused half the class to fail'

• ruggedness

5. hardness, harshness, inclemency, rigor, rigorousness, rigour, rigourousness, severeness, severity, stiffness

excessive sternness

Example Sentences:
'severity of character'
'the harshness of his punishment was inhuman'
'the rigors of boot camp'

• harshness
• inclemency
• rigor
• rigorousness
• rigour
• rigourousness
• severeness
• severity
• stiffness