The Collaborative International Dictionary of English v.0.48:

Resonance \Res"o*nance\ (r?z"?-nans), n. [Cf. F. r['e]sonance,
   L. resonantia an echo.]
   1. The act of resounding; the quality or state of being
      resonant.
      [1913 Webster]

   2. (Acoustics) A prolongation or increase of any sound,
      either by reflection, as in a cavern or apartment the
      walls of which are not distant enough to return a distinct
      echo, or by the production of vibrations in other bodies,
      as a sounding-board, or the bodies of musical instruments.
      [1913 Webster]

   3. (Physics) A phenomenon in which a vibration or other
      cyclic process (such as tide cycles) of large amplitude is
      produced by smaller impulses, when the frequency of the
      external impulses is close to that of the natural cycling
      frequency of the process in that system.

   Note: The shattering of a glass object when impinged upon by
         sound of a certain frequency is one example of this
         phenomenon; another is the very large tides in certain
         basins such as that of the Bay of Fundy, which has a
         natural cycling frequency close to that of the tidal
         cycle.
         [PJC]

   4. (Electronics) An electric phenomenon corresponding to that
      of acoustic resonance, due to the existance of certain
      relations of the capacity, inductance, resistance, and
      frequency of an alternating circuit; the tuning of a radio
      transmitter or receiver to send or detect waves of
      specific frequencies depends on this phenomenon.
      [Webster 1913 Suppl. +PJC]

   Pulmonary resonance (Med.), the sound heard on percussing
      over the lungs.

   Vocal resonance (Med.), the sound transmitted to the ear
      when auscultation is made while the patient is speaking.
      [1913 Webster]

WordNet (r) 3.0 (2006):

resonance
    n 1: an excited state of a stable particle causing a sharp
         maximum in the probability of absorption of electromagnetic
         radiation
    2: a vibration of large amplitude produced by a relatively small
       vibration near the same frequency of vibration as the natural
       frequency of the resonating system
    3: having the character of a loud deep sound; the quality of
       being resonant [syn: plangency, resonance,
       reverberance, ringing, sonorousness, sonority,
       vibrancy]
    4: a relationship of mutual understanding or trust and agreement
       between people [syn: rapport, resonance]
    5: the quality imparted to voiced speech sounds by the action of
       the resonating chambers of the throat and mouth and nasal
       cavities

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "resonance":
   amplitude, antinode, crest, de Broglie wave, diffraction,
   electromagnetic radiation, electromagnetic wave, fluctuation,
   frequency, frequency band, frequency spectrum, guided wave,
   harmonic motion, in phase, interference, libration, light,
   longitudinal wave, mechanical wave, node, nutation, oscillation,
   out of phase, pendulation, period, periodic wave, periodicity,
   radio wave, ray, reinforcement, resonance frequency, seismic wave,
   shock wave, sound wave, surface wave, tidal wave, transverse wave,
   trough, vacillation, vibrancy, vibratility, vibration, wave,
   wave equation, wave motion, wave number, wavelength, wavering