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):

Trapa bicornis
    n 1: water chestnut whose spiny fruit has two rather than 4
         prongs [syn: ling, ling ko, Trapa bicornis]