magic cookie: [UNIX] n. 1. Something passed between routines or programs that enables the receiver to perform some operation;

:magic cookie: [UNIX] n. 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}.
-- The AI Hackers Dictionary