A bunion is really just an enlargement of the joint, typically the big toe joint. Invariably it is mostly bone, but there is often some bursa and soft tissue swelling involved as well.
Hallux Valgus is the term that was traditionally used to describe the angulation of the big toe (hallux) over towards the lessor toes. However, valgus is a term that describes a position in the frontal plane, whereas that angulation of the big toe over towards the lessor toes occurs in the transverse plane, so it should have been called hallux abductus and not hallux valgus. But, as well as abducting in the transverse plane, the hallux does also rotate in the frontal plane, so it does go into valgus in that place.
For the technically correct use of terminology, the correct term should be hallux abducto-valgus (HAV)