Polyfill
In plain terms
Code that adds modern features to older environments lacking them.
Going deeper
A polyfill implements a newer API in JavaScript so older browsers gain the capability natively-missing. It fills feature gaps for compatibility.
Under the hood
Loaded conditionally to avoid bloat; distinct from transpilation, which rewrites syntax rather than adding APIs.
Examples
Promise/fetch polyfills, core-js, supporting legacy browsers, IE compatibility.
Think of it like...
Like an adapter that teaches an old device a new trick it didn't ship with.