And where did it define natural-born citizen?
It does not define any terms used.
They used words everyone understood.
Do you not know what “shall not be infringed” means, either?
The word self defines. The word used by the English is "Subject". The word "citizen" in the modern meaning (member of a nation state) has a specific origin in time and place, and it is easily tracked back to that specific location and time.
It comes from Switzerland, and it's usage as describing a member of a nation state traces back to the Charte des prêtres in 1370.(the old Swiss confederacy foundation document.) In the English of the 1760s, it meant "city-Denizen, or "town-man." We deliberately adopted the Swiss meaning of the word because it was a Swiss man that put the idea of a confederation of independent states into our founders heads.
Finally, several sovereign and independent states may unite themselves together by a perpetual confederacy, without ceasing to be, each individually, a perfect state. They will together constitute a federal republic: their joint deliberations will not impair the sovereignty of each member, though they may, in certain respects, put some restraint on the exercise of it, in virtue of voluntary engagements. A person does not cease to be free and independent, when he is obliged to fulfil engagements which he has voluntarily contracted.
(published in 1758)
Perhaps if you learned more about the natural law foundation of the United States system of governance, you would have a better understanding of how Lincoln damaged our system of governance.