A bunion (hallux valgus) is a deformity characterized by lateral deviation of the great toe, often erroneously described as an enlargement of bone or tissue around the joint at the head of the big toe (metatarsophalangeal joint). As the great toe (hallux) turns in toward the second toe (angulation) the tissues surrounding the joint may become swollen and tender.
The term is used to refer to the pathological bump on the side of the great toe joint. The bump is partly due to the swollen bursal sac and/or an osseous (bony) anomaly on the mesophalangeal joint (where the first metatarsal bone and hallux meet). The larger part of the bump is a normal part of the head of the first metatarsal bone that has tilted sideways to stick out at its top.