Source : Webster's Revised Unabridged Dictionary (1913)

Water flea \Wa"ter flea`\ (Zo["o]l.)
   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.

Source : WordNet®

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}]