any of several perennial water plants of the genus Trapa (family Trapaceae, order Myrtales), native to Europe, Asia, and Africa. The name is also applied to their edible, nutlike fruit.

The water caltrop (T. natans) has submerged leaves that are long, feathery, and rootlike, and floating leaves, in a loose rosette, that are attached to petioles, or leafstalks, 5 to 10 cm (2 to 4 inches) long. The fruit is 2.5 to 5 cm in diameter and usually has four spiny angles.

T. bispinosa, sometimes called Singhara nut, is native to India. The floating leaves, about 5 to 8 cm long, have hairy petioles 10 to 15 cm in length. The fruit is about 2 cm in diameter. T. bicornis, the ling nut, is cultivated in most of East Asia.

The Chinese water chestnut (Eleocharis tuberosus) is a member of the sedge family (Cyperaceae).