Source : Webster's Revised Unabridged Dictionary (1913)

Hardness \Hard"ness\, n. [AS. heardness.]
   1. The quality or state of being hard, literally or
      figuratively.

            The habit of authority also had given his manners
            some peremptory hardness.             --Sir W.
                                                  Scott.

   2. (Min.) The cohesion of the particles on the surface of a
      body, determined by its capacity to scratch another, or be
      itself scratched;-measured among minerals on a scale of
      which diamond and talc form the extremes.

   3. (Chem.) The peculiar quality exhibited by water which has
      mineral salts dissolved in it. Such water forms an
      insoluble compound with soap, and is hence unfit for
      washing purposes.

   Note: This quality is caused by the presence of calcium
         carbonate, causing temporary hardness which can be
         removed by boiling, or by calcium sulphate, causing
         permanent hardness which can not be so removed, but may
         be improved by the addition of sodium carbonate.

Source : WordNet®

hardness
     n 1: the property of being rigid and resistant to pressure; not
          easily scratched; measured on Mohs scale [ant: {softness}]
     2: devoid of passion or feeling [syn: {unfeelingness}, {callousness},
         {insensibility}]
     3: the quality of being difficult to do; "he assigned a series
        of problems of increasing hardness"
     4: excessive sternness; "severity of character"; "the harshness
        of his punishment was inhuman"; "the rigors of boot camp"
        [syn: {severity}, {harshness}, {rigor}, {rigour}, {inclemency},
         {stiffness}]