NOUN (1)

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the Pacific coast of North America; weighs up to six pounds; has siphons that can extend to several feet and cannot be withdrawn into the shell;

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]