The thick soft layer that goes down on the floor first, under a carpet. It does several quiet jobs at once: it makes the carpet feel springy underfoot, it keeps the cold coming up from the floor, it deadens the sound of footsteps, and it takes the wear so the carpet itself lasts much longer. This is British English, and it is the word of the carpet trade — a fitter will ask you what underfelt you want. Americans and increasingly people everywhere say 'carpet padding' or just 'underlay', because these days the layer is often foam or rubber rather than real felt. So 'underfelt' is slightly old-fashioned in the strict sense: the name says felt, but people go on using it even when the material is not felt at all. That kind of drift is very common in English — the name stays after the thing changes.

地毯襯墊（鋪在地毯下面那層）。英式用法；台灣裝潢師傅多半直接講「地毯墊」或用英文 underlay。