The Collaborative International Dictionary of English v.0.48:

Harmonious \Har*mo"ni*ous\ (h[aum]r*m[=o]"n[i^]*[u^]s), a. [Cf.
   F. harmonieux. See Harmony.]
   1. Adapted to each other; having parts proportioned to each
      other; symmetrical.
      [1913 Webster]

            God hath made the intellectual world harmonious and
            beautiful without us.                 --Locke.
      [1913 Webster]

   2. Acting together to a common end; agreeing in action or
      feeling; living in peace and friendship; as, an harmonious
      family.
      [1913 Webster]

   3. Vocally or musically concordant; agreeably consonant;
      symphonious. -- Har*mo"ni*ous*ly, adv. --
      Har*mo"ni*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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