Kale is a form of cabbage (Brassica oleracea Acephala Group) in which the central leaves do not form a head. It is considered to be closer to wild cabbage than most domesticated forms. The species Brassica oleracea contains a wide array of vegetables, including broccoli, cauliflower, and Brussels sprouts.

Until the end of the Middle Ages, kale was the common green vegetable in all of Europe. Curly leaved varieties of cabbage already existed along with flat leafed varieties in Greece, in the fourth century BC. These forms, which were referred to by the Romans as Sabellian kale, are considered to be the ancestors of modern kales.