Source : WordNet®

magic number
     n : the atomic number of an extra stable strongly bound atomic
         nucleus: 2, 8, 20, 28, 50, 82 or 126

Source : Free On-Line Dictionary of Computing

magic number
     
         1. In {source code}, some non-obvious
        constant whose value is significant to the operation of a
        program and that is inserted inconspicuously in-line
        ({hard-coded}), rather than expanded in by a symbol set by a
        commented "#define".  Magic numbers in this sense are bad
        style.
     
        2. A number that encodes critical information used in an
        {algorithm} in some opaque way.  The classic examples of these
        are the numbers used in {hash} or {CRC} functions or the
        coefficients in a {linear congruential generator} for
        {pseudo-random} numbers.  This sense actually predates, and
        was ancestral to, the more common sense 1.
     
        3. Special data located at the beginning of a {binary} data
        file to indicate its type to a utility.  Under {Unix}, the
        system and various {applications programs} (especially the
        {linker}) distinguish between types of executable file by
        looking for a magic number.  Once upon a time, these magic
        numbers were {PDP-11} branch instructions that skipped over
        header data to the start of executable code; 0407, for
        example, was {octal} for "branch 16 bytes relative".  Nowadays
        only a {wizard} knows the spells to create magic numbers.  {MS
        DOS} executables begin with the magic string "MZ".
     
        *The* magic number, on the other hand, is 7+/-2.  The paper
        cited below established the number of distinct items (such as
        numeric digits) that humans can hold in short-term memory.
        Among other things, this strongly influenced the interface
        design of the phone system.
     
        ["The magical number seven, plus or minus two: some limits on
        our capacity for processing information", George Miller, in
        the "Psychological Review" 63:81-97, 1956].
     
        [{Jargon File}]
     
        (2003-07-02)