It was a distinct cultural group who were largely outcasts in both Scotland and Ireland, and lived in certain specific areas of both nations. The majority left for north America because of their treatment in the British Isles.
This is covered in significant detail in "Albion's Seed", among others.
I know what they were. My point is that many people use this and don't seem to properly understand it and appear to be think that it means definition #2 or #3. See the example I gave: https://news.ycombinator.com/item?id=29992408
This is covered in significant detail in "Albion's Seed", among others.