The Collaborative International Dictionary of English v.0.48:

Shellfish \Shell"fish`\, n. (Zool.)
   Any aquatic animal whose external covering consists of a
   shell, either testaceous, as in oysters, clams, and other
   mollusks, or crustaceous, as in lobsters and crabs.
   [1913 Webster]

WordNet (r) 3.0 (2006):

shellfish
    n 1: meat of edible aquatic invertebrate with a shell
         (especially a mollusk or crustacean)
    2: invertebrate having a soft unsegmented body usually enclosed
       in a shell [syn: mollusk, mollusc, shellfish]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "shellfish":
   Dungeness crab, Japanese crab, blue point, clam, coquillage, crab,
   crawdad, crawfish, crayfish, langouste, limpet, littleneck clam,
   lobster, mussel, oyster, periwinkle, prawn, quahog, scallop,
   shrimp, snail, soft-shell crab, steamer, whelk