water chestnut

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A tropical Asian aquatic sedge (Eleocharis dulcis) having cylindrical stems and widely cultivated for its edible corm.
• noun The crisp nutty corm of this plant, used in Asian cooking.
• noun A floating aquatic plant (Trapa natans) native to Eurasia and Africa, cultivated as an ornamental and for its edible four-pronged nutlike fruit, and occurring widely as an aquatic weed.

from The Century Dictionary.

• noun See Trapa.

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun uncountable A particular species, Eleocharis dulcis, of plant.
• noun countable A corm of a plant of this species.
• noun water caltrop

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun edible bulbous tuber of a Chinese marsh plant
• noun a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits
• noun Chinese sedge yielding edible bulb-shaped tubers