A single drop of black blood.
During the slave-years, in Virginia (and possibly in some other states), a Quadroon - someone who was 1/4 black and 3/4 white - was legally Caucasian.
The "One-Drop Rule" was a twentieth century invention, designed to prevent blacks from owning certain real estate.
And yes, there were a lot of "white slaves". Some descended from Irish (and other) slaves, and some descended from a mixing of African slaves with free (or enslaved) white people.
Sally Hemmings (Thos Jefferson's famous slave) was an Octoroon; 1/8 black. Her children were 1/16 black (there was no equivalent term) and were slaves until Jefferson freed them.