What is an American?

Robert Reis gives his answer at MR. Reis is nosing in the right direction (on this particular question), but Harry Laughlin's answer remains the clearest and most comprehensive:

Definition of the American race
Suitable for Legal and Technical Use, -- and for General Understanding

Racially, an American is a Caucasian each of whose ancestral lines traces directly to a member of the foundation racial stock of the American people, or to a race-assimilant thereto who was fully assimilated thereby.

(a) The foundation racial stock of the American people consists in all Caucasians who were inhabitants of the territory of the original United States at the consummation of American independence, September 3, 1783.

(b) A race-assimilant to the foundation racial stock of the American people is a person readily assimilable thereby on account of nearness in blood-kinship thereto; such a person is a Caucasian all of whose ancestral lines trace directly to one or more of those previously established European races which furnished the ethnic elements of the foundation racial stock of the American people.