I am guessing that you have not found a definition of “natural born citizen” in the Constitution, Case Law of Legislation.
And I am guessing you meant,"Constitution, Case Law OR Legislation."
Try this:
U.S. v. Wong Kim Ark's importance is that it is the first case decided by the Supreme Court that attempts to explain the meaning of "natural born citizen" under Article II, Section 1, Clause 5 of the U.S. Constitution. Natural born citizen is similiar to the meaning of what a natural born subject is under Common Law in England. That is one of the reasons why the framers specifically included a grandfather clause (natural born Citizen OR a Citizen of the United States, at the time of adoption of this Constitution). The founding fathers knew that in order to be president, they had to grandfather themselves in because they were British subjects. If they didn't, they could not be President of the U.S. The holding in U.S. v. Wong Kim Ark states that Wong Kim Ark is a native born citizen. If you look at the fact of Wong Kim Ark being born in San Francisco, CA, of Chinese parents, that holding is correct.
In Wong Kim Ark, the court thoroughly discussed “natural born citizen”. And in doing so, Justice Gray quoted directly from the holding in a prior Supreme Court case, Minor v. Happersett. The following passage is a quote from Minor as quoted by Justice Gray in Wong Kim Ark:
” ‘At common law, with the nomenclature of which the framers of the constitution were familiar, it was never doubted that all children born in a country, of parents who were its citizens, became themselves, upon their birth, citizens also. These were natives or natural-born citizens, as distinguished from aliens or foreigners. Some authorities go further, and include as citizens children born within the jurisdiction, without reference to the citizenship of their parents. As to this class there have been doubts, but never as to the first. For the purposes of this case, it is not necessary to solve these doubts. It is sufficient, for everything we have now to consider, that all children, born of citizen parents within the jurisdiction, are themselves citizens.’ Minor v. Happersett (1874) 21 Wall. 162, 166-168.”
Perkins v. Elg's importance is that it actually gives examples of what a Citizen of the U.S. is; what a native born American Citizen is; and what a natural born citizen of the U.S. is. A natural born citizen is a person who is born of two U.S. citizen parents AND born in the mainland of U.S.
The Natural Born Citizen -- The Law