What is a bunyon on your foot? What causes it?

Bunion. A bunion (hallux abducto valgus) is a bony deformity of the great toe characterized by latral deviation of the great toe toward the second toe. This forms an abnormal bony bump at the base of the big toe and an enlargement of the big toe joint. This can be hereditary or from other disease e.g. Arthritis etc.
Bunion. A bunion is a term to describe malalignment of your big toe joint, it appears as a large bump on the outside of the joint, it is a combination of a bony deformity of the first metatarsal bone and soft tissue contraction at the base of the big toe causing it to drift over towards the second toe. Can be caused by improper tight shoes and sometimes genetics.
Painful "bump" A bunion is a progressive disorder that is often described as a bump on the side of the big toe joint which may become painful. The visible bump actually reflects changes in the framework of that joint and possibly adjacent joints. In later stages, the big toe may lean towards the second toe which throws the bones out of alignment – producing the bunion’s “bump” which may lead to pain.