The Constitution does not define the phrase natural-born citizen, and various opinions have been offered over time regarding its precise meaning. A 2011 Congressional Research Service report stated
The weight of legal and historical authority indicates that the term "natural born" citizen would mean a person who is entitled to U.S. citizenship "by birth" or "at birth", either by being born "in" the United States and under its jurisdiction, even those born to alien parents; by being born abroad to U.S. citizen-parents; or by being born in other situations meeting legal requirements for U.S. citizenship "at birth". Such term, however, would not include a person who was not a U.S. citizen by birth or at birth, and who was thus born an "alien" required to go through the legal process of "naturalization" to become a U.S. citizen.
The natural-born-citizen clause has been mentioned in passing in several decisions of the United States Supreme Court and lower courts dealing with the question of eligibility for citizenship by birth, but the Supreme Court has never directly addressed the question of a specific presidential or vice-presidential candidate's eligibility as a natural-born citizen.