Progressive Enhancement/Graceful Degradation for HTML5 and ASP.NET

Can someone give some specific definitions (if there are any) for the following:

• Shims
• Fallbacks
• Polyfills

These are important topics at the moment and these particular terms are being thrown around in the blogosphere rather loosely, from what I can see. I would really like a definitive explanatio开发者_JAVA百科n of the terms and how they relate to each other.

polyfill: A shim that mimics a future API, providing fallback functionality to older browsers.

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

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

http://blog.reybango.com/2010/10/11/how-polyfills-fill-in-the-gaps-to-make-html5-and-css3-usable-today/