A polyfill is code (or a plugin) that would allow you to have some specific functionality only found in "modern" browsers. It allows that feature to also work in other browsers that do not have the support for that functionality built in.

• Polyfills are not part of the HTML5 standard
• Polyfilling is not limited to Javascript

Source

• http://www.programmerinterview.com/index.php/html5/html5-polyfill/