A fellowship is the period of medical training, in the United States and Canada, that a physician, dentist, or veterinarian 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 an attending physician or a consultant physician in the specialist 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.
https://en.wikipedia.org/wiki/Fellowship_(medicine)

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Note added at 26 mins (2023-09-27 14:34:35 GMT)
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residency vs. fellowship
rezydentura (specjalizacja podstawowa) vs. fellowship (specjalizacja szczegółowa)

https://www2.proz.com/personal-glossaries/entry/14440418-res...