yam, common name for some members of the Dioscoreaceae, a family of tropical and subtropical climbing herbs or shrubs with starchy rhizomes often cultivated for food. The largest genus, Dioscorea, is commercially important in East Asia and in tropical America. The thick rhizomes, often weighing 30 lb (13.6 kg) or more, are used for human consumption and for feeding livestock. A number of species of Dioscorea are cultivated for extraction of diosgenin, a female hormone precursor used in the manufacture of the contraceptive pill. In the United States, cultivation of yams for food is restricted to the South, but the wild yam (sometimes used medicinally) is indigenous farther north, and another species, the cinnamon vine, is cultivated as a decorative plant. The sweet potato, which belongs to the morning glory family, is sometimes erroneously called yam. The S African elephant's-foot (Testudinaria elephantipes), also called Hottentot bread and tortoise plant, is sometimes grown in greenhouses; its large rootstock was formerly eaten by the natives. Yams are classified in the division Magnoliophyta, class Liliopsida, order Liliales, family Dioscoreaceae.

yam Tubers of about 10 cultivated species of Dioscorea, most of which are tropical, but 2 of which (D. opposita and D. japonica) are cultivated in temperate regions in Asia. In the USA the name is also applied to the sweet potato (Ipomoea batatas), a somewhat similar but unrelated edible tuber. The yam bean is a name given to the seed of several species of Leguminosae (e.g. Pachyrhizus erosus and Sphenostylis stenocarpa) which also produce edible tubers.

yam Tubers of perennial climbing plants of Dioscorea spp.: D. rotundala white yam, and D. cayenensis, yellow or Guinea yam, water, trifoliate, or Chinese yam. A major food in parts of Africa and also the Far East. A 150‐g portion is a source of vitamins B₁ and C; provides 5 g of dietary fibre; supplies 200 kcal (840 kJ). In the USA sweet potatoes are sometimes called yams.

yam Any of several species of herbaceous vines that grow in warm and tropical regions; also the large, tuberous roots of several tropical species, which are edible. The plant is an annual, with a long, climbing stem, lobed or unlobed leaves and small clusters of greenish, bell-shaped flowers. The sweet potato is also sometimes called a yam. Family Dioscoreaceae; genus Dioscorea.

yam / yam/ • n. 1. the edible starchy tuber of a climbing plant, widely distributed in tropical and subtropical countries. 2. the plant (genus Dioscorea, family Dioscoreaceae) that yields this tuber. 3. a sweet potato. ORIGIN: late 16th cent.: from Portuguese inhame or obsolete Spanish iñame, probably of West African origin.

yam (tuberous root of) species of Dioscorea. XVII (earlier in Eng. writings in various alien forms, e.g. inany, nname, igname). — Pg. inhame or Sp. †igñame (mod. ñame); ult. orig. unkn.

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