The Collaborative International Dictionary of English v.0.48:

Water flea \Wa"ter flea`\ (Zool.)
   Any one of numerous species of small aquatic Entomostraca
   belonging to the genera Cyclops, Daphnia, etc; -- so
   called because they swim with sudden leaps, or starts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water flea
    n 1: minute free-swimming freshwater copepod having a large
         median eye and pear-shaped body and long antennae used in
         swimming; important in some food chains and as intermediate
         hosts of parasitic worms that affect man e.g. Guinea worms
         [syn: cyclops, water flea]
    2: minute freshwater crustacean having a round body enclosed in
       a transparent shell; moves about like a flea by means of
       hairy branched antennae [syn: daphnia, water flea]