Onion is a term used for many plants in the genus Allium. They are known by the common name "onion" but, used without qualifiers, it usually refers to Allium cepa. Allium cepa is also known as the 'garden onion' or 'bulb' onion and 'shallot'.

Allium cepa is known only in cultivation, but related wild species occur in Central Asia. The most closely-related species include Allium vavilovii Popov & Vved. and Allium asarense R.M. Fritsch & Matin from Iran.Grubben, G.J.H. & Denton, O.A. (2004) Plant Resources of Tropical Africa 2. Vegetables. PROTA Foundation, Wageningen; Backhuys, Leiden; CTA, Wageningen. However Zohary and Hopf warn that "there are doubts whether the vavilovii collections tested represent genuine wild material or only feral derivatives of the crop."

Uses

Onions, one of the oldest vegetables known to humankind, are found in a bewildering array of recipes and preparations, spanning almost the totality of the world's cultures; they are nowadays available in fresh, frozen, canned, pickled, powdered, and dehydrated forms. Onions can be used, usually chopped or sliced, in almost every type of food, including cooked foods and fresh salads, and as a spicy garnish; they are rarely eaten on their own but usually act as accompaniment to the main course. Depending on the variety, an onion can be sharp, spicy, tangy and pungent or mild and sweet.

Onions pickled in vinegar are eaten as a snack. These are often served as a side serving in fish and chip shops throughout the United Kingdom. Onions are widely-used in India, and are fundamental to Indian cooking. They are commonly used as a base for curries, or made into a paste and eaten as a main course or as a side dish.

Tissue from onions is frequently used in science education to demonstrate microscope usage, because they have particularly large cells which are readily observed even at low magnifications.

When eaten raw, onions may irritate the stomach. When that happens, milk is effective in neutralizing the effects.fact: date=August 2008

Historical uses

It is thought that bulbs from the onion family have been used as a food source for millennia. In Caananite Bronze Age settlements, traces of onion remains were found alongside fig and date stones dating back to 5000 BC.

The onion is easily propagated, transported and stored. The Ancient Egyptians worshipped it, believing that its spherical shape and concentric rings symbolized eternal life. Onions were even used in Egyptian burials as evidenced by onion traces being found in the eye sockets of Ramesses IV. They believed that if buried with the dead, the strong scent of onions would bring breath back to the dead.