one-drop rule

[wuhn-drop rool]

noun

1. U.S. History.,  a social classification, codified in law in some states during the 20th century, that identifies biracial or multiracial individuals as Black if they have any known Black African ancestry, even from a Black ancestor many generations removed.

Word History and Origins

Origin of one-drop rule¹

First recorded in 1920–25 (as one-drop law ); from the ideology that “one drop” of Black African blood made a person Black