What is the meaning of polyfills in HTML5?

What is the meaning of polyfills in HTML5? I saw this word in many sites about HTML5, e.g. HTML5-Cross-Browser-Polyfills.

So here we're collecting all the shims, fallbacks, and polyfills in order to implant HTML5 functionality in browsers that don't natively support them.

I actually don't understood what is the meaning of polyfills.

Is it a new HTML5 technique or a JavaScript library? I never heard this word before HTML5.

And what is the difference between shims, fallbacks, and polyfills?

A polyfill is a browser fallback, made in JavaScript, that allows functionality you expect to work in modern browsers to work in older browsers, e.g., to support canvas (an HTML5 feature) in older browsers.

It's sort of an HTML5 technique, since it is used in conjunction with HTML5, but it's not part of HTML5, and you can have polyfills without having HTML5 (for example, to support CSS3 techniques you want).

Here's a good post:

http://remysharp.com/2010/10/08/what-is-a-polyfill/

Here's a comprehensive list of Polyfills and Shims:

https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills

First off let's clarify what a polyfil is not: A polyfill is not part of the HTML5 Standard. Nor is a polyfill limited to Javascript, even though you often see polyfills being referred to in those contexts.

The term polyfill itself refers to some code that "allows you to have some specific functionality that you expect in current or “modern” browsers to also work in other browsers that do not have the support for that functionality built in. "

Source and example of polyfill here:

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