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