A bunion is a painful deformity of the bones and joint between the foot and the big toe. Long-term irritation (chronic inflammation) from arthritis, poorly-fitting shoes, or heredity causes the joint to thicken and enlarge. This causes the bones of the big toe to angle in toward and over the second toe, the foot bone (metatarsal) to angle out toward the other foot, and the skin to thicken (callus formation).