(1) A padded jacket, trousers, or other clothing filled with eider down.
(2) More commonly, a duvet is a type of bedding. It consists of a soft flat bag filled with down, feathers, wool, silk or a synthetic, and typically protected with a removable cover, something the way a pillow relates to a pillow case.
In American English, it might be called a comforter, though a comforter is usually has no cover and is often used over a top sheet. Anyway, pretty cushy for backpacking gear.
In Viking times duvets of eider down were used in Norway. Later on the wealthy of Europe used them. Now we have ultralight backpacking quilts.