Source : Free On-Line Dictionary of Computing

Defense Data Network
     
        (DDN) A global communications network serving the US
        Department of Defense.  Composed of {MILNET}, other portions
        of the {Internet}, and classified networks which are not part
        of the {Internet}.  The DDN is used to connect military
        installations and is managed by the {Defense Information
        Systems Agency}.
     
        (1994-12-01)