26 CFR §31.3121(h)-1 — (h)-1 American employer.

1. (a)The term “American employer” means an employer which is (1) the United States or any instrumentality thereof, (2) an individual who is a resident of the United States, (3) a partnership, if two-thirds or more of the partners are residents of the United States, (4) a trust, if all of the trustees are residents of the United States, or (5) a corporation organized under the laws of the United States or of any State. For provisions relating to the terms “State” and “United States”, see § 31.3121(e)-1.
2. (b)For provisions relating to services performed outside the United States by a citizen of the United States as an employee for an American employer, see paragraph (c)(3) of § 31.3121(b)-3 and paragraph (e) of § 31.3121(b)(4)-1.