Source : Webster's Revised Unabridged Dictionary (1913)

Duplex \Du"plex\, v. t. [See {Duplex}, a.] (Teleg.)
   To arrange, as a telegraph line, so that two messages may be
   transmitted simultaneously; to equip with a duplex
   telegraphic outfit.

Duplex \Du"plex\, a. [L., fr. duo two + plicare to fold. See
   {Two}, and {Complex}.]
   Double; twofold.

   {Duplex escapement}, a peculiar kind of watch escapement, in
      which the scape-wheel has two sets of teeth. See
      {Escapement}.

   {Duplex lathe}, one for turning off, screwing, and surfacing,
      by means of two cutting tools, on opposite sides of the
      piece operated upon.

   {Duplex pumping engine}, a steam pump in which two steam
      cylinders are placed side by side, one operating the
      valves of the other.

   {Duplex querela} [L., double complaint] (Eccl. Law), a
      complaint in the nature of an appeal from the ordinary to
      his immediate superior, as from a bishop to an archbishop.
      --Mozley & W.

   {Duplex telegraphy}, a system of telegraphy for sending two
      messages over the same wire simultaneously.

   {Duplex watch}, one with a duplex escapement.

Source : WordNet®

duplex
     adj 1: (used technically of a device or process) having two parts;
            "a duplex transaction"
     2: allowing communication in opposite directions
        simultaneously; "duplex system"; "duplex telephony"

duplex
     n 1: a house with two units sharing a common wall [syn: {duplex
          house}, {semidetached house}]
     2: an apartment having rooms on two floors that are connected
        by a staircase [syn: {duplex apartment}]

Source : Free On-Line Dictionary of Computing

duplex
     
         Used to describe a communications channel
        that can carry signals in both directions, in contrast to a
        {simplex} channel which only ever carries a signal in one
        direction.
     
        If signals can only flow in one direction at a time the
        communications is "{half-duplex}", like a single-lane road with
        traffic lights at each end.  Walkie-talkies with a
        "press-to-talk" button provide half-duplex communications.
     
        If signals can flow in both directions simultaneously the
        communications is "{full-duplex}", like a normal two-lane
        road.  Telephones provide full-duplex communications.
     
        The term "duplex" was first used in wireless, telegraph, and
        telephone communications.  Nearly all communications circuits
        used by computers are two-way, so the term is seldom used.
     
        {(http://www.cit.ac.nz/smac/dc100www/dc_014.htm)}.
     
        (2001-07-21)