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What is a polyglot?

A polyglot is code that runs in multiple programming languages. For example, this one prints Hello, world! in ANSI C, PHP, and Bash:

#define a /*
#<?php
echo "\010Hello, world!\n";// 2> /dev/null > /dev/null \ ;
// 2> /dev/null; x=a;
$x=5; // 2> /dev/null \ ;
if (($x))
// 2> /dev/null; then
return 0;
// 2> /dev/null; fi
#define e ?>
#define b */
#include <stdio.h>
#define main() int main(void)
#define printf printf(
#define true )
#define function
function main()
{
printf "Hello, world!\n"true/* 2> /dev/null | grep -v true*/;
return 0;
}
#define c /*
main
#*/

Here is more information about polyglots.

Polyglots will often require programs to do the same thing (example), but many challenges require that programs do different things in each language (example).