Final answer:
The term for people being defined as black if they had any African ancestry after the US Civil War is the "one-drop rule". This rule contributed to institutionalized racial segregation during the Jim Crow era and significantly impacted American societal views on race.
Step-by-step explanation:
After the US Civil War, the biologizing of race became extreme and people were defined as black if they had a single African ancestor, a criterion known as the "one-drop rule". During the Jim Crow era, this rule was part of a larger set of legislation that further entrenched racial segregation and discrimination. It is significant to acknowledge that in earlier times, there was a significant population with mixed African, European, and Native American ancestry. Over time and especially by the mid-20th century, the US Census Bureau eliminated the use of terms such as ‘mulatto’ which recognized mixed racial heritage, reinforcing a binary categorization of race into solely black or white.
The notion of race derivatives, such as mulattos, quadroons, and octoroons, were once common in the South and illustrated a societal obsession with the degree of black or European ancestry an individual possessed. Although these classifications, along with the one-drop rule, are now outdated, they have left a lasting mark on the social and political landscape of race within America.