From Coastal Wiki
Definition of Gastropod:
organisms belonging to the group Gastropoda - one of the classes of the phyllum Mollusca. This class includes thousands of species of marine snails and sea slugs, as well as freshwater snails and freshwater limpets, and the terrestrial (land) snails and slugs. They generally creep by means of a flat, muscular disk, or foot, on the ventral side of the body. The head usually bears one or two pairs of tentacles.
This is the common definition for Gastropod, other definitions can be discussed in the article