Source : Free On-Line Dictionary of Computing

magic cookie
     
        1. Something passed between routines or programs that enables
        the receiver to perform some operation; a {capability} ticket
        or {opaque identifier}.  Especially used of small data objects
        that contain data encoded in a strange or intrinsically
        machine-dependent way.  E.g. on non-{Unix} {operating system}s
        with a non-byte-stream model of files, the result of "{ftell}"
        may be a magic cookie rather than a byte offset; it can be
        passed to "{fseek}", but not operated on in any meaningful
        way.  The phrase "it hands you a magic cookie" means it
        returns a result whose contents are not defined but which can
        be passed back to the same or some other program later.
     
        2. An in-band code for changing graphic rendition (e.g. inverse
        video or underlining) or performing other control functions.
        Some older terminals would leave a blank on the screen
        corresponding to mode-change magic cookies; this was also
        called a {glitch} (or occasionally a "turd"; compare {mouse
        droppings}).
     
        See also {cookie}.
     
        [{Jargon File}]
     
        (1995-01-25)