By an act of congress in 1790, and revised in 1795, the term has legal meaning.
"The requirements for citizenship and its very definition have changed since the Constitution was ratified in 1788. Congress first recognized the citizenship of children born to U.S. parents overseas on March 26, 1790, under the first naturalization law: "And the children of citizens of the United States that may be born beyond sea, or outside the limits of the United States, shall be considered as natural born citizens."<5><6> Many members of the 1790 Congress had been members of the Constitutional Convention. In addition, George Washington was president of the Constitutional Convention and President of the United States when this bill became law, yet it was not vetoed.
In 1795, the Congress passed the Naturalization Act of 1795 which removed the words "natural born" from the 1790 statute, to state that such children born to citizens beyond the seas are citizens of the U.S., but are not legally to be considered "natural born citizens" of the U.S. This was done to clarify for those living at that time who was and who was not a "natural born citizen" per the framers intent at that time, since the 1790 Act had introduced confusion into that subject in regards to the use of those words in the Constitution. George Washington was also President in 1795, and thus he was aware of this change, and yet did not veto it."
http://en.wikipedia.org/wiki/Natural-born_citizenThere have been many laws passed since then that are relevant to the legal status.
McCain might or might not be 'natural born', Obama, born in Hawaii, most certainly is. The USSC has issued opinions in at least four cases documented on the wiki page that are relevant to this issue.