one drop rule 

When segregation was practiced in the Southern United States a person that was even the smallest percentage black was simply considered black

Barack Obama is technically biracial but according to the one drop rule he would have simply been considered black.

one drop rule by Mr.Juan-derful September 12, 2010

one drop rule 

if you have even a "drop" of blood from a race, it makes you partially that race.

"Dude you cant say the n-word your not black."
"One drop rule nigga im a quarter black!"

one drop rule by enlightend 1 January 5, 2008