Over the years, there have been many derivations of "under coating", but when Jeff refers to this, we're generally referring to "under coating" as that thick gooey stuff that dealers put under cars. The nasty tar-based stuff that never completely dried! That's the only point of contention in labeling the original as "sealant" or "sound deadener", depending where it was used on the car. So, not simply technical terms, but rather describe the type of material used.