There is another reason to use it, said Jennifer L. Hochschild, a professor of government and African-American studies at Harvard. “The court, or some clever clerk, doesn’t really want to use the word white in part because roughly half of Hispanics consider themselves white.” She added, “White turns out to be a much more ambiguous term now than we used to think it was.”

There are a number of terms that refer to various degrees of blackness, both current and out of favor: African-American, mulatto, Negro, colored, octaroon. There are not a lot of options for whites. In Texas, they say Anglo. And there is the pejorative we were so pithily reminded of when a witness in the racially charged George Zimmerman trial said the victim, Trayvon Martin, had called Mr. Zimmerman a “creepy-ass cracker.”

IN the South, I was often asked about my ethnic origins, and I had a ready answer. “My father is from India,” I would recite, phrasing it in such a way as to avoid being mistaken for an American Indian. “And my mom is white.” Almost invariably, if I was speaking to black people, they would nod with understanding. If I was speaking to white people, I would get a puzzled look. “What kind of white?” they would ask. Only when I explained the Norwegian, Scottish and German mix of my ancestry would I get the nod.

I theorized that this was because blacks understood “white” as a category, both historical and contemporary — a coherent group that wielded power and excluded others. Whites, I believed, were less comfortable with that notion.

But Matthew Pratt Guterl, the author of “The Color of Race in America, 1900-1940,” had a different take. “They’re trying to trace your genealogy and figure out what your qualities are,” he said. “They’re looking in your face, they’re looking in the slope of your nose, the shape of your brow. There’s an effort to discern the truth of the matter, because all whitenesses are not equal.” In other words, they weren’t rejecting the category, they were policing its boundaries.

Such racial boundaries have increasingly been called into question in the debate over affirmative action, once regarded as a form of restitution to descendants of slaves, but now complicated by all sorts of questions about who, exactly, is being helped. “What if some of them aren’t poor, what if some of them don’t have American parentage, what if some of them are really stupid?” Ms. Painter, the historian, asked. “There’s all kinds of characteristics that we stuff into race without looking, and then they pop out and we think, ‘I can’t deal with that.’ ”

Doubtless, this society will continue to classify people by race for some time to come. And as we lumber toward justice, some of those classifications remain useful, even separate from other factors like economic class. Caucasian, though? Not so much.