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. It is grown underground by the plant as a vertical shoot that is used for food storage, leading to the possibility of confusion with a tuber, which it is not.
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. 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."
Onions, one of the oldest vegetables known to humankind, are found in a large number of recipes and preparations spanning almost the totality of the world's cultures. They are nowadays available in fresh, frozen, canned, pickled, powdered, chopped, 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 and are referred to simply as "Pickled Onions". Onions are widely-used in India / Pakistan and are fundamental in the local cuisine. 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. Onion is called "Pyaaz" in Urdu,(प्याज़)in Hindi, "Erulli/Ulla gaddi" in Kannada "ulli paya" (ఉల్లిపాయ) or "Erra gadda" (ఎర్ర గడ్డ) in Telugu "Sawala" or "Ulli" in Malayalam , "Kanda" in Marathi and "vengayam" in Tamil.
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.
It is thought that bulbs from the onion family have been used as a food source for millennia. In Bronze Age settlements, traces of onion remains were found alongside fig and date stones dating back to 5000 BC.
However, it is not clear if these were cultivated onions. Archaeological and literary evidence such as the Book of Numbers 11:5 suggests cultivation probably took place around two thousand years later in ancient Egypt, at the same time that leeks and garlic were cultivated. Workers who built the Egyptian pyramids may have been fed radishes and onions.
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.
In ancient Greece, athletes ate large quantities of onion because it was believed that it would lighten the balance of blood. Roman gladiators were rubbed down with onion to firm up their muscles. In the Middle Ages onions were such an important food that people would pay their rent with onions and even give them as gifts. Doctors were known to prescribe onions to facilitate bowel movements and erection, and also to relieve headaches, coughs, snakebite and hair loss. The onion was introduced to North America by Christopher Columbus on his 1492 expedition to Hispaniola. Onions were also prescribed by doctors in the early 1500s to help with infertility in women, and even dogs and cattle and many other household pets. However, recent evidence has shown that dogs, cats, and other animals should not be given onions in any form, due to toxicity during digestion.