Onion, also known as bulb onion or common onion, is a kind of vegetable, which is the most widely planted species in Allium.

The genus also includes several other species, known as onions and cultivated for food, such as Japanese onion (onion), tree onion (A. proliferum) and Canadian onion (Canadian onion). The name "wild onion" is suitable for many species of Allium, but only Aspergillus cepacius is known from cultivation. Although the practice of escaping cultivation has been established in some areas, its original wild form is not clear. Onions are most often biennial or perennial, but are usually considered annual and harvested in their first growing season.

The onion plant has a fan-shaped hollow blue-green leaf. When it reaches the length of a certain day, the bulb at the bottom of the plant begins to expand. The bulb consists of a shortened, compressed underground stem surrounded by fleshy modified scales (leaves) and surrounded by a central bud at the tip of the stem. In autumn (or in spring, if the onion is wintering, in spring), the leaves wither and the outer layer of the bulb becomes dry and brittle. Harvest and dry crops, and onions can be used or stored. Crops are vulnerable to a variety of diseases and insect pests, especially onion flies, onion e insects and various fungi that cause rot. Some varieties of onion, such as onion and potato onion, produce multiple bulbs.

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