Where can I find the definition for national and US national?

United States Code

A good source to use is uscode.house.gov

Title) 8 Ch 12 Sec 1101 (21) (22)

(21) The term “national” means a person owing permanent allegiance to a state (small s state is USA/state of the union).

(22) The term “national of the United States” means (A) a citizen of the United States or (B) a person who, though not a citizen of the United States, owes permanent allegiance to the United States, the (corp.) United States. This is where they capitalize the S in State. The US national is not a US citizen.

Travel.State.Gov Website

Certificates of Non Citizen Nationality
https://travel.state.gov/content/travel/en/legal/travel-legal-considerations/us-citizenship/Certificates-Non-Citizen-Nationality.html

8 U.S. Code § 1401 – Nationals and citizens of United States at birth

https://www.law.cornell.edu/uscode/text/8/1401