Onion's a term used for many plants in the genus Allium. They're known by the common name "onion" but, used without qualifiers, it usually refers to Allium cepa. Allium cepa's also known as the "garden onion" or "bulb" onion. It's grown underground by the plant as a vertical shoot that's used for food storage, leading to the possibility of confusion with a tuber, which it isn't. Allium cepa's known only in cultivation, but related wild species occur in Central Asia. The most closely-related spe… (More on Onions)