Source : Webster's Revised Unabridged Dictionary (1913)

Flatness \Flat"ness\, n.
   1. The quality or state of being flat.

   2. Eveness of surface; want of relief or prominence; the
      state of being plane or level.

   3. Want of vivacity or spirit; prostration; dejection;
      depression.

   4. Want of variety or flavor; dullness; insipidity.

   5. Depression of tone; the state of being below the true
      pitch; -- opposed to {sharpness} or {acuteness}.

Source : WordNet®

flatness
     n 1: the property of having two dimensions [syn: {two-dimensionality},
           {planeness}]
     2: the property of having little or no contrast; lacking
        highlights or gloss [syn: {lusterlessness}, {lustrelessness},
         {mat}, {matt}, {matte}]