Edible bulb, Allium cepa.
Onion (also known as bulb onion, common onion, or garden onion) is an edible bulb, species Allium cepa. Allium cepa is only known in cultivation but related species can be found in Central Asia. Allium contains a number of other "onions" such as Japanese bunching onion, and Egyptian onion, but Allium cepa is by far the most common Allium in cultivation.
Onions are grown for eating, and can be found in a very wide range of foods. Typically the bulb is eaten, but the lower, younger end of the leaf shoots can also be eaten. They are available in yellow, red, and white colors. Onion sizes also vary widely, with the smallest (an inch or less) typically being used for pickling. Young, small onion bulbs are often eaten as "scallions".
Onions are often grown from seed or "set" (small bulbs germinated the previous year). Growers should be careful to avoid bolting which occurs when the onion puts out a flower head before dying.
Use this tag for all questions about growing onions.