S.301 of the Immigration and Nationality Act (INA).
Citizenship is acquired , and "naturally born" is a prima facie assumption.So the question is what is "Unaturally born?"
Many servicemen and women have children abroad foeign postings, and this is how they obtain citizenship. But it would be rather gauche to suggest that hey are NOT naturally born in most instances of citizenship law contests involving the issue ( ie. qualifying for an SS number etc.) Of course one of the areas where that definition is an issue is in Article II , section 1 of the constitution.
There is a reason it is not defined. It simply has not had to be, yet.
Perhaps an Obama case will do so.
The definition cannot be changed by simple passage of a statute by Congress. It means whatever it meant at the time that the Constitution was written.