The Collaborative International Dictionary of English v.0.48:

microphone \mi"cro*phone\ (m[imac]"kr[-o]*f[=o]n), n. [Micro- +
   Gr. fwnh` sound, voice: cf. F. microphone.] (Physics)
   An instrument for converting sounds into electrical signals,
   for the purpose of recording or amplifying the sounds. It
   produces its effects in various ways, as for example by the
   changes of intensity in an electric current, occasioned by
   the variations in the contact resistance of conducting
   bodies, especially of imperfect conductors, under the action
   of acoustic vibrations. Other forms of microphone may use
   changes in capacitance or other phenomena to transduce the
   sounds into electrical signals.

   Note: The electrical signals produced in a microphone may be
         transmitted to recording or amplifying equipment
         through a conducting wire, or by transmission as radio
         waves. The latter method is popular for use in small
         mobile microphones used by performers in plays and
         other entertainment events, at public meetings, and by
         broadcast personnel.
         [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

microphone
    n 1: device for converting sound waves into electrical energy
         [syn: microphone, mike]

The Free On-line Dictionary of Computing (30 December 2018):

microphone

    Any electromechanical device designed to
   convert sound into an electrical signal.

   A microphone converts an acoustic waveform consisting of
   alternating high and low air pressure travelling through the
   air into a voltage.  To do this it uses some kind of pressure
   or movement sensor.  The simplest kind of microphone is
   actually very similar in construction to a loudspeaker.

   The analogue electrical signal can be fed into a computer's
   sound card where it is amplified and sampled to convert it
   into a digital waveform for storage or transmission.

   (2002-11-04)