The Collaborative International Dictionary of English v.0.48:

Eider \Ei"der\, n. [Of Scand. origin, cf. Icel [ae]?r; akin to
   Sw. eider, Dan. ederfugl.] (Zo["o]l.)
   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.
   [1913 Webster]

   Eider down. [Cf. Icel. [ae][eth]ard[=u]n, Sw. eiderd[=u]n,
      Dan. ederduun.] Down of the eider duck, much sought after
      as an article of luxury.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eider
    n 1: duck of the northern hemisphere much valued for the fine
         soft down of the females [syn: eider, eider duck]