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

full-duplex
fdx

    (fdx, from telegraphy) 1. A type of
   duplex communications channel which carries data in both
   directions at once.

   On purely digital connections, full-duplex communication
   requires two pairs of wires.  On analog networks or in
   digital networks using carriers, it is achieved by dividing
   the bandwidth of the line into two frequencies, one for
   sending, and the other for receiving.

   2. An obsolete term for remote echo.

   Compare simplex, half-duplex, double-duplex.

   (2001-07-21)