ei·der / ˈīdər/ • n. (also eider duck) (pl. same or eiders / ˈīdərz/ ) a northern sea duck (genera Somateria and Polysticta), of which the male has mainly black and white plumage with a colored head, and the brown female has soft down feathers that are used to line the nest, in particular the common eider (S. mollissima). ∎ another term for eiderdown.

eider Large, sea duck found in n Europe and North America. Its down is used as a filling for pillows and quilts. In the breeding season, the male grows striking black and white plumage. Family Anatidae; genus Somateria.