As a noun, underlay is the thick soft material laid down on the floor first, before a carpet goes on top of it. You never see it once the carpet is down, but you feel it: it makes the carpet softer to walk on, quieter, warmer, and it makes the carpet last much longer. It is a normal shopping word in Britain, Australia and New Zealand — a carpet shop will ask what underlay you want. American English much more often says 'padding' or 'carpet pad' for the same thing, so this is a word where the two Englishes genuinely part ways. It is usually uncountable ('we need more underlay'), though people do say 'a good underlay' when they mean a type. There is also a verb 'underlay' (to lay something underneath), which is where the noun comes from, but the verb is rare and slightly technical, and it trips people up because its past tense is 'underlaid'. The real trap is a different one, so watch out: 'underlay' also looks exactly like the past tense of the verb 'underlie' — 'the problems that underlay his decision' means 'that were underneath / behind his decision'. Same spelling, completely different word. Only the sentence tells you which one you are looking at.

地毯襯墊、地毯底墊（鋪在地毯下面那層厚軟的材料）。英國、紐澳的裝潢用語；美式英文多半說 padding 或 carpet pad。台灣裝潢師傅一般直接講「地毯襯墊」。