First introduced in colonial Virginia in the early 18th Century as a means of restricting the rights of anyone deemed to be more than 50% Native American, the term only became widespread after the Indian Reorganization Act of 1934.
By then, it had become a mechanism for calculating the amount of federal benefits a tribe could expect to receive, based on its population.
Over time, different tribes have adopted different levels of blood quantum.
Florida's Miccosukee Tribe is among a handful which require members to have 50% tribal blood - or the equivalent of one full blood parent.
At the other end of the spectrum, some Cherokee and Apache tribes require just 6.25% blood quantum. The largest number of tribes, however, require 25% or 12.5%.
