"What is a bunion? A bunion is deformity of the big toe joint, also known as the first metatarsal. When you have a bunion, the first metatarsal bone is more mobile than it should be and will gradually drift away from the second metatarsal. As the bone protrudes more, it will rub on the shoe and this pressure will cause the bone to enlarge. This enlargement is called a bunion."