Source : Webster's Revised Unabridged Dictionary (1913)

Harmonious \Har*mo"ni*ous\, a. [Cf. F. harmonieux. See
   {Harmony}.]
   1. Adapted to each other; having parts proportioned to each
      other; symmetrical.

            God hath made the intellectual world harmonious and
            beautiful without us.                 --Locke.

   2. Acting together to a common end; agreeing in action or
      feeling; living in peace and friendship; as, an harmonious
      family.

   3. Vocally or musically concordant; agreeably consonant;
      symphonious. -- {Har*mo"ni*ous*ly}, adv. --
      {Har*mo"ni*ous*ness}, n.

Source : WordNet®

harmoniousness
     n 1: the property of sounding harmonious [syn: {consonance}]
     2: compatibility in opinion and action [syn: {harmony}]