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]