Source : Webster's Revised Unabridged Dictionary (1913)

Water chestnut \Wa"ter chest"nut\ (Bot.)
   The fruit of {Trapa natans} and {Trapa bicornis}, Old World
   water plants bearing edible nutlike fruits armed with several
   hard and sharp points; also, the plant itself; -- called also
   {water caltrop}.

Source : WordNet®

water chestnut
     n 1: a plant of the genus Trapa bearing spiny four-pronged edible
          nutlike fruits [syn: {water chestnut plant}, {caltrop}]
     2: Chinese sedge yielding edible bulb-shaped tubers [syn: {Chinese
        water chestnut}, {Eleocharis dulcis}]
     3: edible bulbous tuber of a Chinese marsh plant