The Collaborative International Dictionary of English v.0.48:

Graylag \Gray"lag`\, n. (Zool.)
   The common wild gray goose (Anser anser) of Europe,
   believed to be the wild form of the domestic goose. See
   Illust. of Goose.
   [1913 Webster]

WordNet (r) 3.0 (2006):

graylag
    n 1: common grey wild goose of Europe; ancestor of many domestic
         breeds [syn: greylag, graylag, greylag goose,
         graylag goose, Anser anser]