Quine

A Quine is a program that produces its complete source code as its only output. Programs that take input are not considered quines.

a quine that contains no code is ruled out as trivial; in many programming languages executing such a program will output the code (i.e. nothing). Such an empty program once won the "worst abuse of the rules" prize in the Obfuscated C contest.

Quines are named after philosopher Willard Van Orman Quine, who made an extensive study of indirect self-reference. He coined, among others, the following paradox-producing expression, known as Quine's paradox: "'Yields falsehood when preceded by its quotation' yields falsehood when preceded by its quotation."

A quine exists in any programming language that has the ability to output any computable string, as a direct consequence of Kleene's recursion theorem.

An example in colloquial English would be.

"quoted and followed by itself is a quine." 
quoted and followed by itself is a quine. 

An example in C Programming language follows.

/* A simple quine (self-printing program), in standard C. */
/* Note: in designing this quine, we have tried to make the code clear
* and readable, not concise and obscure as many quines are, so that
* the general principle can be made clear at the expense of length.
* In a nutshell: use the same data structure (called "progdata"
* below) to output the program code (which it represents) and its own
* textual representation. */

#include <stdio.h>

void quote(const char *s)
   /* This function takes a character string s and prints the
    * textual representation of s as it might appear formatted
    * in C code. */
{
  int i;

  printf("    \"");
  for (i=0; s[i]; ++i) {
      /* Certain characters are quoted. */
      if (s[i] == '\\')
          printf("\\\\");
      else if (s[i] == '"')
          printf("\\\"");
      else if (s[i] == '\n')
          printf("\\n");
      /* Others are just printed as such. */
      else
          printf("%c", s[i]);
      /* Also insert occasional line breaks. */
      if (i % 48 == 47)
          printf("\"\n    \"");
  }
  printf("\"");
}

/* What follows is a string representation of the program code,
* from beginning to end (formatted as per the quote() function
* above), except that the string _itself_ is coded as two
* consecutive '@' characters. */
const char progdata[] =
  "/* A simple quine (self-printing program), in st"
  "andard C. */\n\n/* Note: in designing this quine, "
  "we have tried to make the code clear\n * and read"
  "able, not concise and obscure as many quines are"
  ", so that\n * the general principle can be made c"
  "lear at the expense of length.\n * In a nutshell:"
  " use the same data structure (called \"progdata\"\n"
  " * below) to output the program code (which it r"
  "epresents) and its own\n * textual representation"
  ". */\n\n#include <stdio.h>\n\nvoid quote(const char "
  "*s)\n /* This function takes a character stri"
  "ng s and prints the\n * textual representati"
  "on of s as it might appear formatted\n * in "
  "C code. */\n{\n int i;\n\n printf(\" \\\"\");\n "
  " for (i=0; s[i]; ++i) {\n /* Certain cha"
  "racters are quoted. */\n if (s[i] == '\\\\')"
  "\n printf(\"\\\\\\\\\");\n else if (s["
  "i] == '\"')\n printf(\"\\\\\\\"\");\n e"
  "lse if (s[i] == '\\n')\n printf(\"\\\\n\");"
  "\n /* Others are just printed as such. */\n"
  " else\n printf(\"%c\", s[i]);\n "
  " /* Also insert occasional line breaks. */\n "
  " if (i % 48 == 47)\n printf(\"\\\"\\"
  "n \\\"\");\n }\n printf(\"\\\"\");\n}\n\n/* What fo"
  "llows is a string representation of the program "
  "code,\n * from beginning to end (formatted as per"
  " the quote() function\n * above), except that the"
  " string _itself_ is coded as two\n * consecutive "
  "'@' characters. */\nconst char progdata[] =\n@@;\n\n"
  "int main(void)\n /* The program itself... */\n"
  "{\n int i;\n\n /* Print the program code, cha"
  "racter by character. */\n for (i=0; progdata[i"
  "]; ++i) {\n if (progdata[i] == '@' && prog"
  "data[i+1] == '@')\n /* We encounter tw"
  "o '@' signs, so we must print the quoted\n "
  " * form of the program code. */\n {\n "
  " quote(progdata); /* Quote all. */\n"
  " i++; /* Skip second '"
  "@'. */\n } else\n printf(\"%c\", p"
  "rogdata[i]); /* Print character. */\n }\n r"
  "eturn 0;\n}\n";

int main(void)
   /* The program itself... */
{
  int i;

  /* Print the program code, character by character. */
  for (i=0; progdata[i]; ++i) {
      if (progdata[i] == '@' && progdata[i+1] == '@')
          /* We encounter two '@' signs, so we must print the quoted
           * form of the program code. */
      {
          quote(progdata);    /* Quote all. */
          i++;                /* Skip second '@'. */
      } else
          printf("%c", progdata[i]);  /* Print character. */
  }
  return 0;
}