Onion, Allium cepa, is an herbaceous biennial in the family Liliaceae grown for its edible bulb. The stem of the plant is a flattened disc at the base and the tubular leaves form a pseudostem where their sheaths overlap. The leaves are either erect or oblique and there are 3–8 per plant.

The onion plant produces pink or white flowers clustered on stalks. The bulbs are formed just above the flattened stem of the plant by overlapping leaves. The bulb is made up of several layers, each corresponding to a leaf. They are generally oval but their shape can be variable and occur in clusters of 3–18 to a plant. The bulb is protected by a membrane that turns into a papery coat. Onion plants can reach a height of 50 cm (20 in) and are grown as annuals, harvested after one growing season.

Onion may also be referred to by cultivar and these include red or purple onion, shallots, and spring onions or scallions. The origin of the onion has not been conclusively determined although it is likely to be somewhere in South East Asia where the gene pool.