A water chestnut is a Chinese vegetable that's commonly used in stir fries. That crisp, pale, mild-tasting morsel you picked up with your chopsticks is probably a water chestnut.
There are at least two different plants that are referred to as water chestnuts — and confusingly, both are frequently used by Chinese cooks. Neither is actually a nut, but one, also called a "water caltrop," is a seed. The water chestnut that's most familiar to North American fans of Chinese food has the Latin name eleocharis dulcis, and it's the white vegetable that stays crunchy even when cooked or canned.