magic cookie n. [Unix; common] 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 OSes with a non-byte-stream
   model of files, the result of `ftell(3)' may be a magic cookie rather
   than a byte offset; it can be passed to `fseek(3)', 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 (see also {cookie}). 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}.


Source: The Jargon File