Merriam-Webster Collegiate Dictionary - water chestnut

 noun  Date: 1854  1. any of a genus (Trapa, especially T. natans of the family Trapaceae, the water-chestnut family) of Old World aquatic herbs sometimes grown as ornamentals; also its edible nutlike spiny fruit  2. a whitish crunchy vegetable used especially in Chinese cooking that is the peeled and often sliced tuber of a sedge (Eleocharis dulcis syn. E. tuberosa) native to Asia but widely cultivated elsewhere; also the tuber or the sedge itself