Any species of sea duck of the genus Somateria, esp.
   Somateria mollissima, which breeds in the northern parts of Europe and
   America, and lines its nest with fine down (taken from its own body)
   which is an article of commerce; -- called also eider duck. The
   American eider (S. Dresseri), the king eider (S. spectabilis), and the
   spectacled eider (Arctonetta Fischeri) are related species.