Source : Webster's Revised Unabridged Dictionary (1913)

Steepness \Steep"ness\, n.
   1. Quality or state of being steep; precipitous declivity;
      as, the steepnessof a hill or a roof.

   2. Height; loftiness. [Obs.] --Chapman.

Source : WordNet®

steepness
     n : the property possessed by a slope that is very steep [syn: {abruptness},
          {precipitousness}] [ant: {gradualness}]