[1] : 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"ol.
   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. aeethard=un, sw. eiderd=un,
      dan. ederduun. down of the eider duck, much sought after
      as an article of luxury.
      1913 webster
see also:
somateria somateria mollissima eider duck s. dresseri s. spectabilis arctonetta fischeri 
eider down 

[2] : WordNet (r) 2.0

eider duck
     n : duck of the northern hemisphere much valued for the fine
         soft down of the females syn: eider
see also:
eider 

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Duck, definition of term: Duck
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Eider >> E Words
Eider, definition of term: Eider
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Duck Soup, definition of term: Duck Soup
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Steamer Duck, definition of term: Steamer Duck
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Wood Duck >> W Words
Wood Duck, definition of term: Wood Duck
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Lame Duck, definition of term: Lame Duck
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Fen Duck, definition of term: Fen Duck
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Bombay Duck, definition of term: Bombay Duck
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