The Collaborative International Dictionary of English v.0.48:

Resonator \Res"o*na`tor\ (-n?`t?r), n. [NL. & G.]
   1. (Acoustics) Anything which resounds; specifically, a
      vessel in the form of a cylinder open at one end, or a
      hollow ball of brass with two apertures, so contrived as
      to greatly intensify a musical tone by its resonance. It
      is used for the study and analysis of complex sounds.
      [1913 Webster]

   2. Anything that resounds or resonates; specif.:
      (a) (Teleg.) An open box for containing a sounder and
          designed to concentrate and amplify the sound.
      (b) (Elec.) Any of various apparatus for exhibiting or
          utilizing the effects of resonance in connection with
          open circuits, as a device having an oscillating
          circuit which includes a helix of bare copper wire, a
          variable number of coils of which can be connected in
          circuit with a condenser and spark gap excited with an
          induction coil. It is used to create high-frequency
          electric brush discharges.
      (c) (Wireless Teleg.) The antenna system and other
          high-frequency circuits of a receiving apparatus.
          [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

resonator
    n 1: a hollow chamber whose dimensions allow the resonant
         oscillation of electromagnetic or acoustic waves [syn:
         resonator, cavity resonator, resonating chamber]
    2: an electrical circuit that combines capacitance and
       inductance in such a way that a periodic electric oscillation
       will reach maximum amplitude [syn: resonator, resonant
       circuit]
    3: any system that resonates