Definitions:

1. a quality of water that contains dissolved mineral salts that prevent soap from lathering noun
   • the costs of reducing hardness depend on the relative amounts of calcium and magnesium compounds that are present
2. the quality of being difficult to do noun
   • They assigned a series of problems of increasing hardness.

Synonyms:

• callosity
• callousness
• insensibility
• ruggedness
• severity
• unfeelingness