The Collaborative International Dictionary of English v.0.48:

Harmonics \Har*mon"ics\ (-[i^]ks), n.
   1. The doctrine or science of musical sounds.
      [1913 Webster]

   2. pl. (Mus.) Secondary and less distinct tones which
      accompany any principal, and apparently simple, tone, as
      the octave, the twelfth, the fifteenth, and the
      seventeenth. The name is also applied to the artificial
      tones produced by a string or column of air, when the
      impulse given to it suffices only to make a part of the
      string or column vibrate; overtones.
      [1913 Webster]

WordNet (r) 3.0 (2006):

harmonics
    n 1: the study of musical sound

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "harmonics":
   accord, accordance, attune, attunement, chime, chiming, concentus,
   concert, concord, concordance, consonance, consonancy, consort,
   diapason, euphony, harmony, heavy harmony, homophony, monochord,
   monody, symphony, synchronism, synchronization, three-part harmony,
   tune, unison, unisonance