Definition - What does Polyfill mean?
A polyfill is a code module or resource that fulfills the need of a developer. The term was coined to describe something that would fill in the gap when a certain platform lacks the functionality that developers expected.
Techopedia explains Polyfill
In many cases, people talk about the polyfill as a kind of shim. This type of code will typically provide changes to an API that can fix problems with older platform or application versions. The idea of using a polyfill supports the more general idea of linking technologies with APIs.
This term comes from a type of spackling compound or paste used to fill holes wood, drywall or plaster walls. The term has somewhat caught on in the Web development community to talk about upgrading functionality for HTML5 or other technologies through adding plug-in code pieces that take the place of native functionality.