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3 definitions found
 for magic number
From WordNet (r) 3.0 (2006) :

  magic number
      n 1: the atomic number of an extra stable strongly bound atomic
           nucleus: 2, 8, 20, 28, 50, 82 or 126

From The Jargon File (version 4.4.7, 29 Dec 2003) :

  magic number
   n.
  
      [Unix/C; common]
  
      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 ({
      hardcoded), 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
  
      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?. Many other kinds of files now have magic numbers somewhere; some
      magic numbers are, in fact, strings, like the ! at the beginning of a
      Unix archive file or the %! leading PostScript files. Nowadays only a {
      wizard knows the spells to create magic numbers. How do you choose a fresh
      magic number of your own? Simple ? you pick one at random. See? It's magic!
  
      4. An input that leads to a computational boundary condition, where
      algorithm behavior becomes discontinuous. Numeric overflows (particularly
      with signed data types) and run-time errors (divide by zero, stack
      overflows) are indications of magic numbers. The Y2K scare was probably the
      most notorious magic number non-incident.
  
      The magic number, on the other hand, is 7?2. See The magical number seven,
      plus or minus two: some limits on our capacity for processing information
      by George Miller, in the Psychological Review 63:81-97 (1956). This classic
      paper 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.
  

From The Free On-line Dictionary of Computing (30 December 2018) :

  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
     pseudorandom 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)
  

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