The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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