I think of "polyfill" as filling many different holes to make it so you can forget about the different cases (browser compatabilities in this case). A polyfill pours concrete over a bumpy surface and makes its smooth. "Shim", to me, is essentially synonymous with monkey patch, i.e. something you shove in there to make something work- shoving a folded napkin under a table leg to make it flat is shimmy to me.