The Collaborative International Dictionary of English v.0.48:

modulator-demodulator \mod"u*la`tor-de`mod"u*la`tor\, n.
   An electronic device that converts electronic signals into
   sound waves, and sound waves into electronic signals, used to
   transmit information between computers by the use of ordinary
   telephone lines; usually called a modem.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

modem \mo"dem\ (m[=o]"d[e^]m), n. [by shortening from
   modulator-demodulator.]
   An electronic device that converts electronic signals into
   sound waves, and sound waves into electronic signals, used to
   transmit information between computers by the use of ordinary
   telephone lines; also called modulator-demodulator; as, the
   latest modems can transmit data at 56,000 baud over a clear
   telephone line. The speed of transmission of information by a
   modem is usually measured in units of baud, equivalent to
   bits per second.
   [PJC]

WordNet (r) 3.0 (2006):

modem
    n 1: (from a combination of MOdulate and DEModulate) electronic
         equipment consisting of a device used to connect computers
         by a telephone line

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

MODEM
       MOdulator DEModulator


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

modem

    (Modulator/demodulator) An
   electronic device for converting between serial data
   (typically EIA-232) from a computer and an audio signal
   suitable for transmission over a telephone line connected to
   another modem.  In one scheme the audio signal is composed of
   silence (no data) or one of two frequencies representing zero
   and one.

   Modems are distinguished primarily by the maximum data rate
   they support.  Data rates can range from 75 bits per second up
   to 56000 and beyond.  Data from the user (i.e. flowing from
   the local terminal or computer via the modem to the telephone
   line) is sometimes at a lower rate than the other direction,
   on the assumption that the user cannot type more than a few
   characters per second.

   Various data compression and error correction algorithms
   are required to support the highest speeds.  Other optional
   features are auto-dial (auto-call) and auto-answer which
   allow the computer to initiate and accept calls without human
   intervention.  Most modern modems support a number of
   different protocols, and two modems, when first connected,
   will automatically negotiate to find a common protocol (this
   process may be audible through the modem or computer's
   loudspeakers).  Some modem protocols allow the two modems to
   renegotiate ("retrain") if the initial choice of data rate is
   too high and gives too many transmission errors.

   A modem may either be internal (connected to the computer's
   bus) or external ("stand-alone", connected to one of the
   computer's serial ports).  The actual speed of transmission
   in characters per second depends not just the modem-to-modem
   data rate, but also on the speed with which the processor can
   transfer data to and from the modem, the kind of compression
   used and whether the data is compressed by the processor or
   the modem, the amount of noise on the telephone line (which
   causes retransmissions), the serial character format
   (typically 8N1: one start bit, eight data bits, no
   parity, one stop bit).

   See also acoustic coupler, adaptive answering, baud
   barf, Bulletin Board System, Caller ID, SoftModem,
   U.S. Robotics, UUCP, whalesong.

   Usenet newsgroup: news:comp.dcom.modems.

   (2002-05-04)