The word “ethnicity” has two somewhat overlapping uses in the U.S. It’s often used in the sense of subrace: a group smaller than a continental scale race. Italians and Jews, for example, are said to be “white ethnics.”
The Census Bureau, however, long declared that race and ethnicity were two qualitatively different things. Thus, those of Hispanic ethnicity could be of any race. (For unexplained reasons, everybody else had only one ethnicity: NonHispanic.) Race was biological, ethnicity cultural.
My definitions have been:
- A racial group is a partly inbred extended biological family.
- An ethnic groups is defined by shared traits that are often passed down within biological families — e.g., language, surname, religion, cuisine, accent, self-identification, historical or mythological heroes, musical styles, etc. — but that don’t have to be. (Thus, you can be adopted into an ethnic group, but not into a racial group.)
What ties together the two meanings of ethnicity (subrace vs. cultural grouping) is the idea that continental-scale races are obviously visually recognizable, while small-scale racial differences may be less visible, but more obviously manifest themselves in cultural behavior. Joey Francis Tribbiani Jr. may not look immediately all that different from a British-American, but if you know his name, that he’s Catholic, that his grandmother speaks Italian, etc., you can probably guess he’s of Italian ancestry.
On the other hand, visual phenotype and cultural behavior traits are both clues to ancestry and genetics, which are highly correlated and not exactly the same. (You have lots of ancestors from whom you didn’t inherit any specific genes, due to shuffling of the DNA deck being rather coarse, so going more than a certain number of generations back you start having ancestors who didn’t contribute anything to your particular DNA.)