Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

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)