Source : Free On-Line Dictionary of Computing

Application Protocol Data Unit
     
         (APDU) A {packet} of data exchanged between two
        {application} programs across a {network}.  This is the
        highest level view of communication in the {OSI} {seven layer
        model} and a single packet exchanged at this level may
        actually be transmitted as several packets at a lower layer as
        well as having extra information (headers) added for {routing}
        etc.
     
        (1995-12-19)