shellfish

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various edible aquatic invertebrate animals having a shell, especially mollusks such as clams and oysters, and crustaceans such as lobsters, crabs, and shrimp.
• noun An edible mollusk, in contrast to a crustacean.
• noun The edible flesh of such animals.

from The Century Dictionary.

• noun An aquatic animal, not a fish, having a shell, and especially one which comes under popular notice as used for food or for ornament.
• noun A crustaceous animal, or crustacean, as a crab, lobster, shrimp, or prawn.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An aquatic invertebrate, such as a mollusc or crustacean, that has a shell.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)
• noun invertebrate having a soft unsegmented body usually enclosed in a shell