Source : Webster's Revised Unabridged Dictionary (1913)

Harmonic \Har*mon"ic\, Harmonical \Har*mon"ic*al\, a. [L.
   harmonicus, Gr. ?; cf. F. harmonique. See {Harmony}.]
   1. Concordant; musical; consonant; as, harmonic sounds.

            Harmonic twang! of leather, horn, and brass. --Pope.

   2. (Mus.) Relating to harmony, -- as melodic relates to
      melody; harmonious; esp., relating to the accessory sounds
      or overtones which accompany the predominant and apparent
      single tone of any string or sonorous body.

   3. (Math.) Having relations or properties bearing some
      resemblance to those of musical consonances; -- said of
      certain numbers, ratios, proportions, points, lines.
      motions, and the like.

   {Harmonic interval} (Mus.), the distance between two notes of
      a chord, or two consonant notes.

   {Harmonical mean} (Arith. & Alg.), certain relations of
      numbers and quantities, which bear an analogy to musical
      consonances.

   {Harmonic motion},

Harmonic \Har*mon"ic\, n. (Mus.)
   A musical note produced by a number of vibrations which is a
   multiple of the number producing some other; an overtone. See
   {Harmonics}.

Source : WordNet®

harmonic
     adj 1: of or relating to harmony as distinct from melody and
            rhythm; "subtleties of harmonic change and tonality"-
            Ralph Hill [ant: {nonharmonic}]
     2: of or relating to the branch of acoustics that studies the
        composition of musical sounds; "the sound of the
        resonating cavity cannot be the only determinant of the
        harmonic response"
     3: relating to vibrations that occur as a result of vibrations
        in a nearby body; "sympathetic vibration" [syn: {sympathetic}]
     4: involving or characterized by harmony [syn: {consonant}, {harmonical},
         {harmonized}, {harmonised}, {in harmony}]

harmonic
     n : a tone that is a component of a complex sound