The onion (Allium cepa), which is also known as the bulb onion, common onion and garden onion, is the most widely cultivated species of the genus Allium. The genus Allium also contains a number of other species variously referred to as onions and cultivated for food, such as the Japanese bunching onion (A. fistulosum), Egyptian onion (A. ×proliferum), and Canada onion (A. canadense). The name “wild onion” is applied to a number of Allium species.

The vast majority of cultivars of A. cepa belong to the ‘common onion group’ (A. cepa var. cepa) and are usually referred to simply as ‘onions’. The ‘Aggregatum group’ of cultivars (A. cepa var. aggregatum) includes both shallots and potato onions