26 CFR 31.3121(e)-1: (e)-1 State, United States, and citizen.

(a) When used in the regulations in this subpart, the term “State” includes the District of Columbia, the Commonwealth of Puerto Rico, the Virgin Islands, the Territories of Alaska and Hawaii before their admission as States, and (when used with respect to services performed after 1960) Guam and American Samoa.

(b) When used in the regulations in this subpart, the term “United States”, when used in a geographical sense, means the several states (including the Territories of Alaska and Hawaii before their admission as States), the District of Columbia, the Commonwealth of Puerto Rico, and the Virgin Islands. When used in the regulations in this subpart with respect to services performed after 1960, the term “United States” also includes Guam and American Samoa when the term is used in a geographical sense. The term “citizen of the United States” includes a citizen of the Commonwealth of Puerto Rico or the Virgin Islands, and, effective January 1, 1961, a citizen of Guam or American Samoa.