Definition of "eiderdown"

A great sea lifted us high and, crashing down with a deafening roar, carried us swiftly along on light foam as soft as eiderdown.

1929, Robert Dean Frisbee, The Book of Puka-Puka, Eland, published 2019, page 80

The landlord was near spraining his wrist, and I told him for heaven’s sake to quit—the bed was soft enough to suit me, and I did not know how all the planing in the world could make eider down of a pine plank.

1851 November 14, Herman Melville, “chapter 3”, in Moby-Dick; or, The Whale, 1st American edition, New York, N.Y.: Harper & Brothers; London: Richard Bentley

I entered. It was a very small room, overcrowded with furniture of the style which the French know as Louis Philippe. There was a large wooden bedstead on which was a billowing red eiderdown, and there was a large wardrobe, a round table, a very small washstand, and two stuffed chairs covered with red rep.

1919, W[illiam] Somerset Maugham, “chapter 11”, in The Moon and Sixpence, [New York, N.Y.]: Grosset & Dunlap Publishers […]

She jerked up in bed, clutching the eiderdown; her heart beat. What could it be?

1920, Katherine Mansfield [pseudonym; Kathleen Mansfield Murry], “Revelations”, in Bliss and Other Stories, London: Constable & Company, published 1920, page 263