https://en.wikipedia.org/wiki/Fellowship_(medicine)

A fellowship is the period of medical training in the United States and Canada that a physician or dentist may undertake after completing a specialty training program (residency). During this time (usually more than one year), the physician is known as a fellow. Fellows are capable of acting as attending physician or consultant physician in the generalist field in which they were trained, such as internal medicine or pediatrics. After completing a fellowship in the relevant sub-specialty, the physician is permitted to practice without direct supervision by other physicians in that sub-specialty, such as cardiology or oncology.