Source : Free On-Line Dictionary of Computing

half-duplex
     
         (hdx, from {telegraphy}) 1. A type of
        communication channel using a single circuit which can carry
        data in either direction but not both directions at once.
     
        Compare: {simplex}, {full-duplex}.
     
        2. An obsolete term for {local echo}.
     
        (2001-07-21)