A chemical compound with a pH significantly below 7 (typically below 5.5 pH) that can be foamed onto a surface.
Foam application of an acid enhances its cleaning action by increasing the contact time of the acid to the surface to be cleaned.
Acid products are used to remove stone type build up (milk, beer, etc.) which produces a more brilliant finish on stainless steel in addition to the cleaning action. It also removes calcium deposits and hard water scale build ups.
Supply Sanitation Systems markets a replacement acid cleaner that is so safe you can put your hand in it without harm.