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Edible starchy tuber
the large tuberous starchy root eaten as a staple food

Yam is the common name for some plant species in the genus Dioscorea (family Dioscoreaceae) that form edible tubers (some other species in the genus being toxic). Yams are perennial herbaceous vines native to Africa, Asia, and the Americas and cultivated for the consumption of their starchy tubers in many temperate and tropical regions. The tubers themselves, also called "yams", come in a variety of forms owing to numerous cultivars and related species.

A monocot related to lilies and grasses, yams are vigorous herbaceous, perennially growing vines from a tuber. Some 870 species of yams are known, However, in Portuguese, this name commonly refers to the taro plant (Colocasia esculenta) from the genus Colocasia, as opposed to Dioscorea. The main derivations borrow from verbs meaning "to eat". In Australia, the tubers of the Microseris lanceolata, or yam daisy, were a staple food of Aboriginal Australians in some regions. In New Zealand, oca (Oxalis tuberosa) is typically referred to as "yam". In Malaysia and Singapore, taro (Colocasia esculenta) is referred to as "yam". In Africa, South and Southeast Asia as well as the tropical Pacific islands Amorphophallus paeoniifolius is grown and known as "elephant foot yam".

Yams are native to Africa, Asia, and the Americas.

D. rotundata, the white yam, and D. cayenensis, the yellow yam, are native to Africa. They are the most important cultivated yams. In the past, they were considered as two separate species, but most taxonomists now regard them as the same species. Over 200 varieties between them are cultivated. White yam tuber is roughly cylindrical in shape, the skin is smooth and brown, and the flesh is usually white and firm. Yellow yam has yellow flesh, caused by the presence of carotenoids. It looks similar to the white yam in outer appearance; its tuber skin is usually a bit firmer and less extensively grooved. The yellow yam has a longer period of vegetation and a shorter dormancy than white yam. The Kokoro variety is important in making dried yam chips. They are large plants; the vines can be as long as The tubers most often weigh about each, but can weigh as much as After 7 to 12 months' growth, the tubers are harvested. In Africa, most are pounded into a paste to make the traditional dish of "pounded yam", known as Iyan.

D. alata, called purple yam (not to be confused with the Okinawan purple "yam", which is a sweet potato), greater yam, ube, winged yam, water yam, and (ambiguously) white yam,

D. polystachya, Chinese yam, is native to China. The Chinese yam plant is somewhat smaller than the African, with the vines about long. It is tolerant to frost and can be grown in much cooler conditions than other yams. It is also grown in Korea and Japan. It was introduced to Europe in the 19th century, when the potato crop there was falling victim to disease, and is still grown in France for the Asian food market. The tubers are harvested after about 6 months of growth. Some are eaten right after harvesting and some are used as ingredients for other dishes, including noodles, and for traditional medicines.

D. esculenta, the lesser yam, was one of the first yam species cultivated. It is native to Southeast Asia and is the third most commonly cultivated species there, although it is cultivated very little in other parts of the world. Its vines seldom reach more than in length and the tubers are fairly small in most varieties. The tubers are eaten baked, boiled, or fried much like potatoes. Because of the small size of the tubers, mechanical cultivation is possible, which along with its easy preparation and good flavor, could help the lesser yam to become more popular in the future. Research on propagation of this subspecies to alleviate the threat from wild harvest has been successful.

D. japonica known as East Asian mountain yam, yamaimo, or Japanese mountain yam is a type of yam (Dioscorea) native to Japan (including Ryukyu and Bonin Islands), Korea, China, Taiwan, and Assam. D. japonica is used for food. Jinenjo, also called the wild yam, is a related variety of Japanese yam that is used as an ingredient in soba noodles.

Yams in West Africa are typically harvested by hand, using sticks, spades, or diggers.

Unlike cassava, most varieties of edible, mature, cultivated yam do not contain toxic compounds. However, there are exceptions. Bitter compounds tend to accumulate in immature tuber tissues of white and yellow yams. These may be polyphenols or tannin like compounds. Wild forms of bitter yams (D. dumetorum) do contain some toxins, such as dihydrodioscorine, that taste bitter, hence are referred to as bitter yam. Bitter yams are not normally eaten except at times of desperation in poor countries and in times of local food scarcity. They are usually detoxified by soaking in a vessel of salt water, in cold or hot fresh water or in a stream. The bitter compounds in these yams are water soluble alkaloids which, on ingestion, produce severe and distressing symptoms. Severe cases of alkaloid intoxication may prove fatal. Aerial or potato yams (D. bulbifera) have antinutritional factors. In Asia, detoxification methods, involving water extraction, fermentation, and roasting of the grated tuber, are used for bitter cultivars of this yam. The bitter compounds in yams also known locally as air potato include diosbulbin and possibly saponins, such as diosgenin. In Indonesia, an extract of air potato is used in the preparation of arrow poison.

Raw yam has only moderate nutrient density, with appreciable content (10% or more of the Daily Value, DV) limited to potassium, vitamin B6, manganese, thiamin, dietary fiber, and vitamin C (table). But raw yam has the highest potassium levels amongst the 10 major staple foods of the world (see nutritional chart). Yam supplies 118 calories per 100 grams. Yam generally has a lower glycemic index, about 54% of glucose per 150 gram serving, compared to potato products. The protein content and quality of roots and tubers is lower than other food staples, with the content of yam and potato being around 2% on a fresh weight basis. Yams, with cassava, provide a much greater proportion of the protein intake in Africa, ranging from 5.9% in East and South Africa to about 15.9% in humid West Africa. As a relatively low protein food, yam is not a good source of essential amino acids. Experts emphasize the need to supplement a yam dominant diet with more protein rich foods to support healthy growth in children. Yam is an important dietary element for Nigerian and West African people. It contributes more than 200 calories per person per day for more than 150 million people in West Africa, and is an important source of income. Yam is an attractive crop in poor farms with limited resources. It is rich in starch, and can be prepared in many ways. It is available all year round, unlike other, unreliable, seasonal crops. These characteristics make yam a preferred food and a culturally important food security crop in some sub Saharan African countries.

The following table shows the nutrient content of yam and major staple foods in a raw harvested form on a dry weight basis to account for their different water contents. Raw forms, however, are not edible and cannot be digested. These must be sprouted, or prepared and cooked for human consumption. In sprouted or cooked form, the relative nutritional and antinutritional contents of each of these staples is remarkably different from that of raw form of these staples.

In the Philippines, the purple ube species of yam (D. alata), is eaten as a sweetened dessert called ube halaya, and is also used as an ingredient in another Filipino dessert, halo halo. It is also used as a popular ingredient for ice cream.

In Vietnam, yams are used to prepare dishes such as canh khoai mỡ or canh khoai từ. This involves mashing the yam and cooking it until very well done. The yam root was traditionally used by peasants in Vietnam to dye cotton clothes throughout the Red River and Mekong delta regions as late as the mid 20th century, and is still used by others in the Sapa region of northern Vietnam.

In Indonesia, the same purple yam is used for preparing desserts. This involves mashing the yam and mixing it with coconut milk and sugar. White and off white fleshed yams are cut in cubes, cooked, lightly fermented, and eaten as afternoon snacks.

An exception to the cooking rule is the mountain yam (Dioscorea polystachya), known as nagaimo and can be further classified into ichōimo (lit. 'ginkgo leaf yam'; kanji: 銀杏芋), or yamatoimo (lit. Yamato yam; kanji: 大和芋), depending on the root shape. Mountain yam is eaten raw and grated, after only a relatively minimal preparation: the whole tubers are briefly soaked in a vinegar water solution to neutralize irritant oxalate crystals found in their skin. The raw vegetable is starchy and bland, mucilaginous when grated, and may be eaten plain as a side dish, or added to noodles. Another variety of yam, jinenjo, is used in Japan as an ingredient in soba noodles. In Okinawa, purple yams (Dioscorea alata) are grown. This purple yam is popular as lightly deep fried tempura, as well as being grilled or boiled. Additionally, the purple yam is a common ingredient of yam ice cream with the signature purple color. Purple yam is also used in other types of traditional wagashi sweets, cakes, and candy.

In central parts of India, the yam is prepared by being finely sliced, seasoned with spices, and deep fried. In Southern India, the vegetable is a popular accompaniment to rice dishes and curry. The purple yam, D. alata, is also eaten in India, where it is also called the violet yam. Species may be called by the regional name "taradi", which can refer to D. belophylla, Dioscorea deltoidea, and D. bulbifera. Digging and selling taradi is a major source of income in the region of Palampur.

Dioscorea root is traditionally eaten on Māgh Sankrānti (a midwinter festival) in Nepal. It is usually steamed and then cooked with spices.

Yam is, along with cassava and taro, a staple food, and is consumed boiled, roasted in a lovo, or steamed with fish or meat in curry sauce or coconut milk and served with rice. The cost of yam is higher due to the difficulty in farming and relatively low volume of production.

Yam powder is available in the West from grocers specializing in African products, and may be used in a similar manner to instant mashed potato powder, although preparation is a little more difficult because of the tendency of the yam powder to form lumps. The powder is sprinkled onto a pan containing a small amount of boiling water and stirred vigorously. The resulting mixture is served with a heated sauce, such as tomato and chili, poured onto it. Skinned and cut frozen yams may also be available from specialty grocers.

The tubers of certain wild yams, including a variant of 'Kokoro' yam and other species of Dioscorea, such as Dioscorea nipponica, are a source for the extraction of diosgenin, a sapogenin steroid. Such preparations were used in early combined oral contraceptive pills. The unmodified steroid has estrogenic activity.

Historical records in West Africa and of African yams in Europe date back to the 16th century. Yams were taken to the Americas through precolonial Portuguese and Spanish on the borders of Brazil and Guyana, followed by a dispersion through the Caribbean. Yams are used in Papua New Guinea, where they are called kaukau. Their cultivation and harvesting is accompanied by complex rituals and taboos. The coming of the yams (one of the numerous versions from Maré) is described in Pene Nengone (Loyalty Islands of New Caledonia).

A yam festival is usually held in the beginning of August at the end of the rainy season. People offer yams to gods and ancestors first, before distributing them to the villagers. This is their way of giving thanks to the spirits above them. The New Yam Festival celebrates the main agricultural crop of the Igbos, Idomas, and Tivs. The New Yam Festival, known as Orureshi in Owukpa in Idoma west and Ima Ji, Iri Ji or Iwa Ji in Igbo land, is a celebration depicting the prominence of yam in social and cultural life. The festival is prominent among southeastern states and major tribes in Benue State, mainly around August. The Igbo people accord special respect to yam to the extent that no one eats the newly harvested yam until the New Yam celebrations / feast is marked. It is called Iri ji ọhụrụ. People return to their various communities for the celebrations.