The Collaborative International Dictionary of English v.0.48:

Gastropoda \Gas*trop"o*da\, n. pl., [NL., fr. Gr. ?, ?, stomach
   + -poda.] (Zool.)
   One of the classes of Mollusca, of great extent. It includes
   most of the marine spiral shells, and the land and
   fresh-water snails. 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. See
   Mollusca. [Written also Gasteropoda.]
   [1913 Webster]

   Note: The Gastropoda are divided into three subclasses; viz.:
         (a) The Streptoneura or Dioecia, including the
         Pectinibranchiata, Rhipidoglossa, Docoglossa, and
         Heteropoda. (b) The Euthyneura, including the
         Pulmonata and Opisthobranchia. (c) The Amphineura,
         including the Polyplacophora and Aplacophora.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Gastropoda
    n 1: snails and slugs and their relatives [syn: Gastropoda,
         class Gastropoda, Gasteropoda, class Gasteropoda]