The Collaborative International Dictionary of English v.0.48:

oscillator \os"cil*la`tor\ ([o^]s"s[i^]l*l[=a]`t[~e]r), n.
   1. One that oscillates; specif.: (Electronics) Any device or
      circuit for producing electric oscillations, whether of
      current or voltage; esp., an apparatus for generating
      electric waves in a system of wireless telegraphy.

   Note: Oscillators are essential components of radio
         transmission devices and digital computers, as well as
         many other types of electronic device. In computers the
         oscillator provides the voltage impulses which permit
         information bits to be transferred between parts of the
         computer in a defined sequence.
         [Webster 1913 Suppl. +PJC]

   2. (Mech.) An instrument for measuring rigidity by the
      torsional oscillations of a weighted wire.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

oscillator
    n 1: generator that produces sonic oscillations or alternating
         current