The one-drop theory (or one-drop rule) is the colloquial term for the standard, found throughout the USA, that holds that a person with even one drop of non-white ancestry should be classified as "colored", especially for the purposes of laws forbidding inter-racial marriage. This standard has also been applied to people with Native American ancestry.

One-drop theory is still influential in the USA - by de facto American color standards, a multiracial person with black heritage is considered black unless they declare themselves otherwise, identifying instead as white, mixed-race or Native American, for example (different color standards can be seen in countries such as Brazil). These standards are widely rejected in the Latino community in the USA, the majority of which is mixed race.

The theory was codified in law in some states, although the expression colored was often used, at least acknowledging the presence of non-black ancestry. For example, as cited in the Loving v. Virginia decision[1] (http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&navby=case&vol=388&invol=1&friend=oyez), Virginia law (Racial Integrity Law of 1924) held that for the purposes of laws forbidding inter-racial marriage, "Every person in whom there is ascertainable any Negro blood shall be deemed and taken to be a colored person, and every person not a colored person having one fourth or more of American Indian blood shall be deemed an American Indian; except that members of Indian tribes existing in this Commonwealth having one fourth or more of Indian blood and less than one sixteenth of Negro blood shall be deemed tribal Indians."