Definition of "shellfish"

A fisheries and colloquial term for an aquatic invertebrate having an inner or outer shell, such as a mollusc or crustacean, especially when edible.  examples

A culinary and nutritional term for several groups of non-piscine, non-tetrapod, aquatic animals that are used as a food source. The term often exclusively refers to edible aquatic crustaceans, bivalve mollusks and cephalopod mollusks; but sometimes echinoderms may be included as well.  examples