In which US Code is US Citizen defined?

The term "United States citizen" is defined in 8 U.S. Code § 1401, which outlines the conditions under which a person is considered a citizen of the United States at birth. This includes individuals born in the U.S., those born to U.S. citizen parents abroad, and certain other specific circumstances. Additionally, citizenship is addressed in various contexts throughout Title 8 of the U.S. Code, which pertains to Immigration and nationality.