Yam is the common name for some plant species in the genus Dioscorea that form edible tubers. Yams are perennial herbaceous vines cultivated for the consumption of their starchy tubers in many temperate and tropical regions, especially in West Africa, South America and the Caribbean, Asia, and Oceania. The tubers themselves, also called "yams", come in a variety of forms owing to numerous cultivars and related species.
Yams were independently domesticated on three different continents: Africa (Dioscorea rotundata), Asia (Ube), and the Americas (Dioscorea trifida).
Note that in North America, the sweet potato is sometimes referred to as a "yam." The two plants are quite unrelated, but their tubers are both large, soft, and sweet.