The Collaborative International Dictionary of English v.0.48:

Harmonic \Har*mon"ic\ (h[aum]r*m[o^]n"[i^]k), Harmonical
\Har*mon"ic*al\ (-[i^]*kal), a. [L. harmonicus, Gr. "armoniko`s;
   cf. F. harmonique. See Harmony.]
   1. Concordant; musical; consonant; as, harmonic sounds.
      [1913 Webster]

            Harmonic twang! of leather, horn, and brass. --Pope.
      [1913 Webster]

   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.
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   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.
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   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, the motion of the point A, of the foot of
      the perpendicular PA, when P moves uniformly in the
      circumference of a circle, and PA is drawn perpendicularly
      upon a fixed diameter of the circle. This is simple
      harmonic motion. The combinations, in any way, of two or
      more simple harmonic motions, make other kinds of harmonic
      motion. The motion of the pendulum bob of a clock is
      approximately simple harmonic motion.

   Harmonic proportion. See under Proportion.

   Harmonic series or Harmonic progression. See under
      Progression.

   Spherical harmonic analysis, a mathematical method,
      sometimes referred to as that of Laplace's Coefficients,
      which has for its object the expression of an arbitrary,
      periodic function of two independent variables, in the
      proper form for a large class of physical problems,
      involving arbitrary data, over a spherical surface, and
      the deduction of solutions for every point of space. The
      functions employed in this method are called spherical
      harmonic functions. --Thomson & Tait.

   Harmonic suture (Anat.), an articulation by simple
      apposition of comparatively smooth surfaces or edges, as
      between the two superior maxillary bones in man; -- called
      also harmonia, and harmony.

   Harmonic triad (Mus.), the chord of a note with its third
      and fifth; the common chord.
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The Collaborative International Dictionary of English v.0.48:

Harmonic \Har*mon"ic\ (h[aum]r*m[o^]n"[i^]k), 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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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 harmonics
    3: 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"
    4: relating to vibrations that occur as a result of vibrations
       in a nearby body; "sympathetic vibration" [syn: harmonic,
       sympathetic]
    5: involving or characterized by harmony [syn: consonant,
       harmonic, harmonical, harmonized, harmonised]
    n 1: a tone that is a component of a complex sound
    2: any of a series of musical tones whose frequencies are
       integral multiples of the frequency of a fundamental

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "harmonic":
   AF, accordant, according, assonant, assonantal, attuned,
   audio frequency, blended, blending, chiming, concordant, consonant,
   flageolet tone, fluctuant, fluctuating, fluctuational, frequency,
   fundamental, fundamental tone, harmonic tone, harmonious,
   harmonizing, homophonic, in accord, in chorus, in concert,
   in concord, in sync, in tune, in unison, intonation, libratory,
   monodic, monophonic, monotone, monotony, musical, nutational,
   oscillating, oscillatory, overtone, partial, partial tone,
   pendular, pendulous, periodic, pitch, resonant, symphonic,
   symphonious, synchronized, synchronous, tone, tonelessness, tuned,
   unisonant, unisonous, vacillating, vacillatory, vibratile,
   vibrating, vibratory, wavering