quine

/kwi:n/ [from the name of the logician Willard V. Quine, via Douglas Hofstadter] n. A program that generates a copy of its own source text as its complete output. Devising the shortest possible quine in some given programming language is a common hackish amusement. Here is one classic quine:

     ((lambda (x)
       (list x (list (quote quote) x)))
      (quote
         (lambda (x)
           (list x (list (quote quote) x)))))

This one works in LISP or Scheme. It's relatively easy to write quines in other languages such as Postscript which readily handle programs as data; much harder (and thus more challenging!) in languages like C which do not. Here is a classic C quine for ASCII machines:

     char*f="char*f=%c%s%c;main()
     {printf(f,34,f,34,10);}%c";
     main(){printf(f,34,f,34,10);}

For excruciatingly exact quinishness, remove the interior line breaks. Some infamous Obfuscated C Contest entries have been quines that reproduced in exotic ways.