What is "one-drop rule"

n. The notion that one drop of black blood (i.e., any African ancestry at all) makes a person black.

The one-drop rule is a social and legal principle of racial classification that was historically prominent in the United States asserting that any person with even one ancestor of sub-Saharan-African ancestry ("one drop" of black blood) is considered black ( Negro in historical terms). This concept evolved over the course of the 19th century and became codified into law in the 20th century. It was associated with the principle of "invisible blackness" and is an example of hypodescent, the automatic assignment of children of a mixed union between different socioeconomic or ethnic groups to the group with the lower status.

The same principle may be applied, or has been applied in opposite ways in different cultures, as is the case in Brazil vs. the United States.