One who devotes himself to some specialty; as, a medical specialist, one who devotes himself to diseases of particular parts of the body, as the eye, the ear, the nerves, etc.
The physician who provides expertise and care in a particular area; i.e., surgeon, oncologist (cancer doctor), dermatologist, allergist.
Doctors who focus on certain parts of the body or diseases. These doctors have many years of training in their specialty areas. Cardiologists (heart), oncologists (cancer), and rheumatologists (arthritis) are all specialists.