From Longman Dictionary of Contemporary Englishunderlayun‧der‧lay /ˈʌndəleɪ $ -ər-/ noun [countable, uncountable] DHHthick material that is put between a carpet and the floor
Examples from the Corpus
underlay• Everything has an underlay of deceit.• Something to do with a Bob Geldof feature needing an underlay.• These are padded mini-mattresses covered with smooth plastic, but you can make your own from rubberised carpet underlay.• And you can't top that smell-the sweet, dry, rubbery, wet, woolly smell of fresh underlay.• Good carpet needs a good underlay.• In conjunction with these offers, there is free fitting and rubber underlay on any carpet from stock at over £4.99 per square yard.• Some carpets have an integral foam backing as part of the underlay.• Use underlay under woven carpet, or jute-backed tufted carpet. 