An ethnic group is a group of people who identify with one another, or are so identified by others, on the basis of a boundary that distinguishes them from other groups. This boundary may take any of a number of forms -- racial, cultural, linguistic, economic, religious, political -- and may be more or less porous. Because of this boundary, members of an ethnic group are often presumed to be culturally or biologically similar, although this is not in fact necessarily the case.
Another characteristic of ethnic groups is continuity in time, that is, a history and a future as a people. This is achieved through the intergenerational transmission of common language, institutions and traditions. It is important to consider this characteristic of ethnic groups if we are to distinguish them from a group of individuals who share a common characteristic, such as ancestry, in a specific point in time. On the political front, ethnic groups are distinguished from nation-states by the former's lack of sovereignty.
Do Americans constitute a separate ethnonational group, after more than two centuries of independent statehood and almost four centuries of continuous history in their homeland? My inclination is to say that they do. If so, what sort of relationship do they share with other post-British settler cultures, like the English Canadians, the Australians, and the New Zealanders? Do Americans living outside of their homeland form a diaspora?