WordNet (r) 3.0 (2006):

dongle
    n 1: (computer science) an electronic device that must be
         attached to a computer in order for it to use protected
         software

The Jargon File (version 4.4.7, 29 Dec 2003):

dongle
 /dong'gl/, n.

    1. [now obs.] A security or copy protection device for proprietary
    software consisting of a serialized EPROM and some drivers in a D-25
    connector shell, which 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
    startup and at programmed intervals thereafter, and terminate if it does
    not respond with the dongle's programmed validation code. Thus, users can
    make as many copies of the program as they want but must pay for each
    dongle. The first sighting of a dongle was in 1984, associated with a
    software product called PaperClip. The idea was clever, but it was
    initially a failure, as users disliked tying up a serial port this way. By
    1993, dongles would typically pass data through the port and monitor for 
    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.
    These devices have become rare as the industry has moved away from
    copy-protection schemes in general.

    2. By extension, any physical electronic key or transferable ID required
    for a program to function. Common variations on this theme have used
    parallel or even joystick ports. See dongle-disk.

    3. An adaptor cable mating a special edge-type connector on a PCMCIA or
    on-board Ethernet card to a standard 8p8c Ethernet jack. This usage seems
    to have surfaced in 1999 and is now dominant. Laptop owners curse these
    things because they're notoriously easy to lose and the vendors commonly
    charge extortionate prices for replacements.

    [Note: in early 1992, advertising copy from Rainbow Technologies (a
    manufacturer of dongles) included a claim that the word derived from ?Don
    Gall?, allegedly the inventor of the device. The company's receptionist
    will cheerfully tell you that the story is a myth invented for the ad copy.
    Nevertheless, I expect it to haunt my life as a lexicographer for at least
    the next ten years. :-( ?ESR]


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

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)