In the noun form of the term, a "national" is a person who is considered under the legal protection of a country, but not necessarily a citizen. National status generally applies to someone who lived in places acquired by the U.S. before the date of acquisition. A person can be a national-at-birth under a similar set of rules for a natural-born citizen. U.S. nationals must go through the same processes as an immigrant to become a full citizen. U.S. nationals are not considered natural-born after becoming a citizen.
Example of a Federal Statute defining National
The term "national" means a person owing permanent allegiance to a state.[ 8 USCS § 1101]