The Collaborative International Dictionary of English v.0.48:

Ptarmigan \Ptar"mi*gan\, n. [Gael. tarmachan; cf. Ir. tarmochan,
   tarmonach.] (Zool.)
   Any grouse of the genus Lagopus, of which numerous species
   are known. The feet are completely feathered. Most of the
   species are brown in summer, but turn white, or nearly white,
   in winter.
   [1913 Webster]

   Note: They chiefly inhabit the northern countries and high
         mountains of Europe, Asia, and America. The common
         European species is Lagopus mutus. The Scotch grouse,
         red grouse, or moor fowl (Lagopus Scoticus), is
         reddish brown, and does not turn white in winter. The
         white, or willow, ptarmigan (Lagopus albus) is found
         in both Europe and America.
         [1913 Webster]

WordNet (r) 3.0 (2006):

ptarmigan
    n 1: large Arctic and subarctic grouse with feathered feet and
         usually white winter plumage