Source : Webster's Revised Unabridged Dictionary (1913)

Shellfish \Shell"fish`\, n. (Zo["o]l.)
   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.

Source : WordNet®

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}]
     [also: {shellfishes} (pl)]