Use shellfish to refer to crustaceans and mollusks, especially the ones people commonly eat. Crabs, mussels, and shrimp are all shellfish.

If you serve your guests a shellfish stew, they'll know it's full of creatures that originally had shells or hard exoskeletons. In most kitchens and restaurants, the term includes marine animals such as lobsters and oysters, and freshwater species like crayfish too. Fishery regulators only use shellfish for mollusks, including clams and mussels. The funny thing is that shellfish aren't fish at all — they're more closely related to arachnids and insects!

Definitions of shellfish
  1. noun
    invertebrate having a soft unsegmented body usually enclosed in a shell
    synonyms: mollusc, mollusk
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    burrowing marine mollusk
    gastropod, univalve
    a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes
    chiton, coat-of-mail shell, polyplacophore, sea cradle
    primitive elongated bilaterally symmetrical marine mollusk having a mantle covered with eight calcareous plates
    bivalve, lamellibranch, pelecypod
    marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together
    cephalopod, cephalopod mollusk
    marine mollusk characterized by well-developed head and eyes and sucker-bearing tentacles
    tooth shell, tusk shell
    any of various seashore mollusks having a tapering tubular shell open at each end and a foot pointed like a spade for burrowing
    abalone, ear-shell
    any of various large edible marine gastropods of the genus Haliotis having an ear-shaped shell with pearly interior
    any of various edible tropical marine gastropods of the genus Strombus having a brightly-colored spiral shell with large outer lip
    freshwater or marine or terrestrial gastropod mollusk usually having an external enclosing spiral shell
    any of various terrestrial gastropods having an elongated slimy body and no external shell
    any of several creeping marine gastropods with a spirally coiled shell: whelks; tritons; moon shells; neritids
    Ancylus fluviatilis, freshwater limpet, river limpet
    minute conical gastropod superficially resembling a limpet but living and feeding on freshwater plants
    nudibranch, sea slug
    any of various marine gastropods of the suborder Nudibranchia having a shell-less and often beautifully colored body
    Aplysia punctata, sea hare
    naked marine gastropod having a soft body with reduced internal shell and two pairs of ear-like tentacles
    bubble shell
    marine gastropod mollusk having a very small thin shell
    any member of the genus Physa
    cowrie, cowry
    any of numerous tropical marine gastropods of the genus Cypraea having highly polished usually brightly marked shells
    aplacophoran, solenogaster
    deep-water wormlike mollusks lacking calcareous plates on the body but having fine slimy spicules on the covering mantle
    burrowing marine mollusk living on sand or mud; the shell closes with viselike firmness
    common edible, burrowing European bivalve mollusk that has a strong, rounded shell with radiating ribs
    marine mollusks having a rough irregular shell; found on the sea bed mostly in coastal waters
    ark shell
    marine bivalve mollusk having a heavy toothed shell with a deep boat-like inner surface
    blood clam
    red-blooded clam
    marine or freshwater bivalve mollusk that lives attached to rocks etc.
    escallop, scallop, scollop
    edible marine bivalve having a fluted fan-shaped shell that swim by expelling water from the shell in a series of snapping motions
    marine bivalve that bores into rock or clay or wood by means of saw-like shells
    chambered nautilus, nautilus, pearly nautilus
    cephalopod of the Indian and Pacific oceans having a spiral shell with pale pearly partitions
    dibranch, dibranchiate, dibranchiate mollusk
    cephalopods having two gills
    a cephalopod with eight arms but lacking an internal shell
    cephalopods having eight short tentacles plus two long ones
    type of:
    any animal lacking a backbone or notochord; the term is not used as a scientific classification
  2. noun
    meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)
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    black marine bivalves usually steamed in wine
    huitre, oyster
    edible body of any of numerous oysters
    flesh of either hard-shell or soft-shell clams
    common edible European bivalve
    crab, crabmeat
    the edible flesh of any of various crabs
    crawdad, crawfish, crayfish, ecrevisse
    tiny lobster-like crustaceans usually boiled briefly
    mollusk with a low conical shell
    flesh of a lobster
    crayfish, langouste, rock lobster, spiny lobster
    warm-water lobsters without claws; those from Australia and South Africa usually marketed as frozen tails; caught also in Florida and California
    escallop, scallop, scollop
    edible muscle of mollusks having fan-shaped shells; served broiled or poached or in salads or cream sauces
    oysters Rockefeller
    oysters spread with butter and spinach and seasonings and baked on the half shell
    blue point, bluepoint
    oysters originally from Long Island Sound but now from anywhere along the northeastern seacoast; usually eaten raw
    hard-shell clam, quahaug, quahog, round clam
    Atlantic coast round clams with hard shells; large clams usually used for chowders or other clam dishes
    long-neck clam, soft-shell clam, steamer, steamer clam
    a clam that is usually steamed in the shell
    blue crab
    Atlantic crab; most common source of fresh crabmeat
    crab legs
    legs of especially Alaska king crabs
    soft-shell crab, soft-shelled crab
    freshly molted crab with new shell still tender and flexible
    Japanese crab
    crabmeat usually canned; from Japan
    Alaska crab, Alaska king crab, Alaskan king crab, king crab
    meat of large cold-water crab; mainly leg meat
    Dungeness crab
    flesh of Cancer magister (Dungeness crab)
    stone crab
    pale flesh with delicate texture and flavor; found in Florida but now very rare
    American lobster, Maine lobster, Northern lobster
    flesh of cold-water lobsters having large tender claws; caught from Maine to the Carolinas
    European lobster
    similar to but smaller than American lobsters
    Norwegian lobster, langoustine, scampo
    caught in European waters; slenderer than American lobster
    lobster tail
    lobster tail meat; usually from spiny rock lobsters
    sea scallop
    muscle of large deep-water scallops
    bay scallop
    muscle of small choice shallow-water scallops
    type of:
    edible fish (broadly including freshwater fish) or shellfish or roe etc
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