May I suggest that a dictionary may not be the place to look for the best possible definition of the four terms that concern you? As Omi and Winant point out in _Racial Formation in the United States_, "race" is a slippery term that is constantly being redefined. Moreover, the day-to-day meaning of race is quite different for different groups of people. And don't be fooled into thinking that there is somehow a "scientific definition" of any of these terms.
You need to look at the historical contexts in which the terms arose. The current use of "African American," for example, dates from the middle to late 1980s. I believe that the introduction of this term was a very deliberate and positive attempt to "demystify" race, by making black people in the United States analogous to any other ethnic group -- like Italian Americans -- rather than contrasting "black" people (as a race) to "white" people (as a race).
Nonetheless, the realities of race persist in this country, and the group known as African Americans, or black Americans (or, in earlier times, American Negroes or colored people) continue to be defined as a "race." What makes someone an African American or black American? I think the one-drop rule still prevails -- an African American is any person with the least bit of recognizable black African ancestry (or any person, even if he or she could pass for "white," who acknowledges having black African ancestry). Others may wish to try to disentangle "black American " from "African American," based on whether one's forebears were enslaved people in the U.S. or more recent immigrants from Africa or other parts of the Americas. I'm not sure if that is worth the effort.
In this country, the whole notion of "race" exists to distinguish black people from the majority of Americans. It developed as a means to justify slavery and to easily identify people who might be fugitive slaves. Given the definition of "African American" I suggest above, "race" (at least in the United States) would have to be defined as
"Race" is a social category related to "class," but which functions to allow the poorest or most deprived person of the dominant race to feel superior to all members of the subordinate race. In the U.S., racial solidarity serves to blur class lines within the dominant group, and has generally gotten in the way of class-based alliances.
- a form of social stratification,
- based on heritable, physical features that serve as visible cues,
- perpetuated by social disapproval of inter-group marriage or mating, and
- leading to the assignment of "mixed-race" progeny to the subordinate group.
"Ethnic group" I won't try to tackle in any detail. Members of a given ethnic group share aspects of a certain range of sub-cultures, often (but not always) based on myths built around common ancestry in a distant homeland. "Race" shares many of the characteristics of "ethnicity" (especially in terms of shared sub-cultures), but represents a much deeper social cleavage -- often erroneously ascribed to biological differences.
Thanks for your attention.
Institute of the History of Medicine
The Johns Hopkins University