A bunion is a deformity that usually occurs at the head of the first of five long bones (the metatarsal bones) that extend from the arch and connect to the toes. The first metatarsal bone is the one that attaches to the big toe.

The big toe is forced in toward the rest of the toes, causing the head of the first metatarsal bone to jut out and rub against the side of the shoe; the underlying tissue becomes inflamed and a painful bump forms. As this bony growth develops, the bunion is formed as the big toe is forced to grow at an increasing angle towards the rest of the toes.

A bunion may also develop in the bone that joins the little toe to the foot (the fifth metatarsal bone), in which case it is known as a bunionette or tailor's bunion.

Bunions often develop from wearing narrow, high-heeled shoes with pointed toes, which puts enormous pressure on the front of the foot and causes the foot and toes to rest at unnatural angles.
Injury in the joint may also cause a bunion to develop over time.

Genetics play a factor in 10% to 15% of all bunion problems; one inherited deformity, hallux valgus, causes the bone and joint of the big toe to shift and grow inward, so that the second toe crosses over it. Flat feet, gout, and arthritis increase the risk for bunions.