It has been defined many times by many reputable legal sources and by the common law of England. Natural born citizen means born under the jurisdiction of a sovereign and into the allegiance of the same. It means native born. It means born on soil owned and/or controlled by the sovereign. Its meaning was well understood at the time the constitution was adopted. The meaning has been forgotten perhaps over the centuries but it was not lost.
It is not defined by the Constitution, and as you know, international law is sadly wanting an authoritative interpreter. And, as I have said, we must remember that a Constitution is an act of positive law, and the US Constitution was at bottom agreement between several political entities, formerly subject to the King in Parliament of Great Britain and Ireland but now trying to establish a government that would protect their common interests. Of course they built with the bricks lying at hand but we must not suppose that there is more than the foundation of that government, nothing so logical and consistent as, say. the Napoleonic code. So, I merely say what I think. and as know know quite a bit about the history of that time, I am unmoved by attempts to make metaphysical what is merely physics. History is quite clear that without the person of George Washington standing in front of them, the greatest man of the age according to King George himself, the Constitution would not have been written, the new government of the Union would not have taken shape, and lord knows what might have happened if Washington had died that August he went under the knife. So, again, knowing I reduce the matter to my formula: Article II says simply: George Washington, or a man very, very much like him will be President and no foreign prince will be allowed to occupy the office. The second of these was necessary to keep the United States from becoming like every other state a monarchy. The words of certain clauses are but instrumental to the achievement of that end.