They are most properly defined colloquially as discrete and insular minorities, i.e., a group which possesses immutable or highly visible traits. This would include Jews, Gypsies, and other peoples.
I recall visiting Trakai Castle in Lithuania, a nation that is fairly homogenous (with Russian transplants from the Soviet days). Yet, there is a small community that lives near that castle, called the Karaites. The Karaites are ethnic Karaims, a peculiar Turkic community with its own religion. They were brought to Trakai in the 14th and 15th century by Lithuanian King Vytautas to protect the castle -- and only about 65 of them still remain in the town. Yet, Trakai is their heartland to this day and they continue to preserve their traditions there. I stopped at a Karaite restaurant and I sampled some of their food.
In sum, the Karaites are neither immigrants nor expats. They are native born non-ethnic Lithuanians, with a 500-year-old footprint in the country. So, like Jews, Gypsies, and some others, the Karaites are a discrete and insular minority. There may be a scientific name for such communities. It is an interesting topic.
The Hungarians are also an interesting people, with their own unique genetic profile, living amidst a sea of Germanic, Baltic, and Slavic people.