What is a “Prolyfill”?

A “Prolyfill” or “forward polyfill” is similar to a Polyfill but instead of providing a feature which is already implemented by some browsers or part of the specification, it enables a new feature which will probably be implemented by a browser in the future. If a Prolyfill gets a great deal of attention from web developers, browser makers may think of implementing it.

UPDATE: Feb 2017

It seems the official therm for prolyfill is now “speculative polyfill”.