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]

The Collaborative International Dictionary of English v.0.48:

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.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

duplex \du"plex\, n. [See Duplex, a.]
   1. something which is duplex; -- used mostly in reference to
      a living unit, such as an apartment, in a building having
      two similar living units.
      [PJC]

   2. (Biology, Genetics) a double-stranded region in a nucleic
      acid molecule. See deoxyribonucleic acid.
      [PJC]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "duplex":
   Janus-like, ambidextrous, apartment house, bifacial, bifold,
   biform, bifurcated, bilateral, binary, binate, biparous,
   bipartisan, bipartite, bivalent, condominium, conduplicate,
   cooperative apartment house, dichotomous, disomatous, double,
   double-barreled, double-faced, duadic, dual, dualistic, duple,
   duplex house, duplicate, duplicated, dyadic, flats, geminate,
   geminated, high-rise apartment building, identical, matched,
   second, secondary, tenement, twain, twin, twinned, two, two-faced,
   two-level, two-ply, two-sided, two-story, twofold



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

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://cit.ac.nz/smac/dc100www/dc_014.htm).

   (2001-07-21)