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.

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 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]
      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]