Head injury: Damage to any of the structures of the head as a result of trauma. While the term  head injury  is most often used to refer to an injury to the brain, head injuries may also involve the bones, muscles, blood vessels, skin, and other organs of the face or head. A head injury does not always mean that there is an associated brain injury. Most head injuries are caused by blows to the head from numerous causes including motor vehicle accidents and falls. Head injuries are one of the most common causes of death and disability in the United States.