Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {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.

Source : WordNet®

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