\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.