Underlay or underlayment"Underlayment." The Oxford English Dictionary. 2nd ed. 1989. generally refers to a thin layer of cushioning, made of materials such as sponge rubber, foam, felt or crumb rubber, which is laid beneath carpeting to provide comfort underfoot, to reduce wear on the carpet, and to provide insulation against sound, moisture and heat."Underlay." The Oxford English Dictionary. 2nd ed. 1989. In general it's a layer which is underneath another layer. Underlay thus is also used to d… (More on Underlay)