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. Some older terminals would leave a blank on the screen corresponding to mode-change magic cookies; this was also called a glitch.
See also cookie.