The definition of a crustacean is an arthropod with a hard outer shell and jointed limbs that usually lives in aquatic environments.
A lobster is an example of a crustacean.
crustacean definition by Webster's New World
any of a subphylum (Crustacea) of arthropods, including shrimps, crabs, barnacles, and lobsters, that usually live in the water and breathe through gills: they have a hard outer shell and jointed appendages
Origin: Modern Latin Crustacea ; from crustaceus, having a crust or shell ; from Classical Latin crusta, crust
crustacean definition by American Heritage Dictionary
Any of various predominantly aquatic arthropods of the class Crustacea, including lobsters, crabs, shrimps, and barnacles, characteristically having a segmented body, a chitinous exoskeleton, and paired, jointed limbs.
Of or belonging to the Crustacea.
Origin: From New Latin Crūstācea, class name, neuter pl. of crūstāceus, hard-shelled, from Latin crūsta, shell; see kreus- in Indo-European roots.
Any of various widespread arthropods of the class Crustacea that live mostly in water and have a hard shell, a segmented body, and jointed appendages. Crustaceans include crabs, lobsters, shrimp, barnacles, and copepods.