A "natural born citizen" is a person who is entitled under the Constitution or laws of the United States to citizenship “at birth” or “by birth.”
The term includes
(1) a person who is born in the United States, including its territories and possessions and the District of Columbia, and who is subject to its jurisdiction, that is, not born to foreign diplomats or to hostile occupying forces;
(2) a person who is born abroad to two U.S. citizens; and
(3) a person who is born abroad to one U.S. citizen, if that citizen parent has met U.S. residency requirements.
Barack Obama is a natural born citizen because he was born in Hawaii, one of the United States, and was subject to its jurisdiction at the time of his birth.
John McCain is a natural born citizen because he was born abroad to two citizens, and was born in the Canal Zone, a United States possession.
The Supreme Court has held that a person born of aliens in the United States is a natural born citizen, since that child "is as much a citizen as the natural-born child of a citizen." Wong Kim Ark