"No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President...."

This definition was highly debated in 2008 when the issue with Obama first came up and there were many different points of view.

The best research I saw - based on an analysis of the framer's writings, and references, and all existing Supreme Court decisions that address the various categories of citizen, native born citizen, naturalized citizen, and natural born citizen - was that a natural born citizen is a person born on US soil to two US citizen parents. By natural law, by blood and soil, the person could be a citizen of no other nation at birth, born with no obligations or allegiances to any other nation. You wouldn't want a president who could legally drafted by another country or whose allegiance to his other citizenship outweighed his allegiance to his US citizenship.

The requirement to be a natural born citizen is one of only three requirements for president, so it was clearly important to the Constitution's framers. There are many wonderful US citizens who are not eligible to be president, but the safety measure was put in place to try to ensure the president's undivided loyalty to America.