The Collaborative International Dictionary of English v.0.48:

Duplex \Du"plex\, a. [L., fr. duo two + plicare to fold. See
   Two, and Complex.]
   1. Double; twofold.
      [1913 Webster]

   2. (Computers) organized so that data may be transmitted in
      two opposite directions over the same channel; -- of
      communications channels, such as data transfer lines
      between computers.
      [PJC]

   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.

   half duplex (Computers)
      (a) arranged so that the information may be transmitted in
          both directions, but only in one direction at a time;
          -- of communications channels between computers;
          contrasted with full duplex(a).
      (b) arranged so that the information transmitted to the
          remote computer also appears on the local terminal; --
          of communications channels between computers;
          contrasted with full duplex(b).

   full duplex, (Computers)
      (a) arranged so that the information may be transmitted in
          both directions simultaneously; -- of communications
          channels between computers; contrasted with half
          duplex(a).
      (b) arranged so that the information transmitted to the
          remote computer does not appear on the local terminal;
          -- of communications channels between computers;
          contrasted with half duplex(b).
          [1913 Webster]