In web development, a polyfill (or polyfiller) is downloadable code which provides facilities that are not built into a web browser.
For example, many features of HTML5 are not supported by versions of Internet Explorer older than version 8 or 9, but can be used by web pages if those pages install a polyfill. Web shims like HTML5 Shiv are a related concept.

Polyfills can also be used to add entirely new functionality to browsers. For instance, BrowserID relies on a Javascript API which (as of mid-2012) is not supported in any browser and must be provided via a polyfill.[3]

The term "polyfiller" is inspired by Polyfilla, the name of a well established brand of wall filler (UK) or spackling paste (US) sold in the UK.

source: WikiPedia