The term Onion does not actually refer to a singular vegetable, but to many vegetable plants in the Genus Allium.
Onions grow underground, and are a vertical shoot used by the plant for food storage. Do not confuse them with tubers however; onions are not tubers.
Considered one of humankind's oldest vegetables, onions are found in culinary use by cultures the entire world over. They can be used in many ways; fresh, frozen, powdered, dehydrated, chopped, canned and pickled. Onions are usually used as an extra added to foods, or as a garnish, but are rarely eaten raw.
Image: Credit: Fir0002/Flagstaffotos, License: GNU Free Documentation License