Source : Webster's Revised Unabridged Dictionary (1913)

Highness \High"ness\, n. [AS. he['a]hnes.]
   1. The state of being high; elevation; loftiness.

   2. A title of honor given to kings, princes, or other persons
      of rank; as, His Royal Highness. --Shak.

Source : WordNet®

Highness
     n 1: (Your Highness or His Highness of Her Highness) title used
          to address a royal person
     2: the condition of being high or lofty [syn: {loftiness}]
        [ant: {lowness}]