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Any species of seaduck of the genus Somateria, especially. Somateria mollissima, which breeds in the northern parts of Europe and America, and lines its nest with beautiful down (taken from its own body) which is an article of commerce; called alsoeider duck. The American eider (S. Dresseri), the kingeider (S. Spectabilis), and the spectacledeider (Arctonetta Fischeri) are related species. Eider down. [Cf. Icel. Aeoardun, Sw. Eiderdun, Dan. Ederduun] Down of the eider duck, many sought after as an article of luxury.
Origin: Of Scand. Origin, cf. Icel aer; akin to Sw. Eider, Dan. Ederfugl.
Source: Websters Vocabulary