The Collaborative International Dictionary of English v.0.48:

Amphipoda \Am*phip"o*da\, n. pl. [NL., fr. Gr. 'amfi` + ?, ?
   foot.] (Zool.)
   A numerous group of fourteen -- footed Crustacea, inhabiting
   both fresh and salt water. The body is usually compressed
   laterally, and the anterior pairs or legs are directed
   downward and forward, but the posterior legs are usually
   turned upward and backward. The beach flea is an example. See
   Tetradecapoda and Arthrostraca.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Amphipoda
    n 1: small flat-bodied semiterrestrial crustaceans: whale lice;
         sand-hoppers; skeleton shrimp [syn: Amphipoda, order
         Amphipoda]