A bunion is an unnatural, bony hump that forms at the base of the big toe where it attaches to the foot. Often, the big toe deviates toward the other toes. When this occurs, the base of the big toe pushes outward on the first metatarsal bone -- which is the bone directly behind the big toe -- forming a bunion. If this happens on the little toe and fifth metatarsal, it's called a bunionette.
Because a bunion occurs at a joint, where the toe bends in normal walking, your entire body weight rests on the bunion at each step. Bunions can be extremely painful. They're also vulnerable to excess pressure and friction from shoes and can lead to the development of calluses. Read more about causes and treatments for bunions.