A bivalve mollusk of many kinds, especially those that are
   edible; as, the long clam (Mya arenaria), the quahog or round clam
   (Venus mercenaria), the sea clam or hen clam (Spisula solidissima), and
   other species of the United States. The name is said to have been given
   originally to the Tridacna gigas, a huge East Indian bivalve.