Source : WordNet®

dongle
     n : (computer science) an electronic device that must be
         attached to a computer in order for it to use protected
         software

Source : Free On-Line Dictionary of Computing

dongle
     
         /dong'gl/ (From "dangle" - because it dangles off
        the computer?)
     
        1.  A security or {copy protection} device for
        commercial {microcomputer} programs that must be connected to
        an {I/O port} of the computer while the program is run.
        Programs that use a dongle query the port at start-up and at
        programmed intervals thereafter, and terminate if it does not
        respond with the expected validation code.
     
        One common form consisted of a serialised {EPROM} and some
        drivers in a {D-25} connector shell.
     
        Dongles attempt to combat {software theft} by ensuring that,
        while users can still make copies of the program (e.g. for
        {backup}), they must buy one dongle for each simultaneous use
        of the program.
     
        The idea was clever, but initially unpopular with users who
        disliked tying up a port this way.  By 1993 almost all dongles
        passed data through transparently while monitoring for their
        particular {magic} codes (and combinations of status lines)
        with minimal if any interference with devices further down the
        line.  This innovation was necessary to allow {daisy-chained}
        dongles for multiple pieces of software.
     
        In 1998, dongles and other copy protection systems are fairly
        uncommon for {Microsoft Windows} software but one engineer in
        a print and {CADD} bureau reports that their {Macintosh}
        computers typically run seven dongles: After Effects, Electric
        Image, two for Media 100, Ultimatte, Elastic Reality and CADD.
        These dongles are made for the Mac's daisy-chainable {ADB}
        port.
     
        The term is used, by extension, for any physical electronic
        key or transferable ID required for a program to function.
        Common variations on this theme have used the {parallel port}
        or even the {joystick} port or a {dongle-disk}.
     
        An early 1992 advertisment from Rainbow Technologies (a
        manufacturer of dongles) claimed that the word derived from
        "Don Gall", the alleged inventor of the device.  The company's
        receptionist however said that the story was a myth invented
        for the ad.
     
        [{Jargon File}]
     
        (1998-12-13)
     
        2. A small adaptor cable that connects, e.g. a {PCMCIA}
        {modem} to a telephone socket or a PCMCIA {network card} to an
        {RJ45} {network cable}.
     
        (2002-09-29)