The Collaborative International Dictionary of English v.0.48:

Geoduck \Ge"o*duck\, n. [American Indian name.] (Zool.)
   A gigantic clam (Glycimeris generosa) of the Pacific coast
   of North America, highly valued as an article of food.
   [1913 Webster]

WordNet (r) 3.0 (2006):

geoduck
    n 1: a large edible clam found burrowing deeply in sandy mud
         along the Pacific coast of North America; weighs up to six
         pounds; has siphons that can extend to several feet and
         cannot be withdrawn into the shell