Why was the first year of postgraduate training once called an internship in North America?

The term "internship" was historically used in North America to describe the first year of postgraduate training because it represented a distinct period of supervised hospital work immediately following medical school, during which a physician gained clinical experience before potentially moving into a specialized residency program.1

It is important to clarify that this terminology has evolved; in modern North American medical education, this initial year is generally no longer distinguished from the broader period of residency. Today, the training is typically viewed as a continuous postgraduate process where physicians seek further education to qualify as specialists or to pursue higher academic degrees.1