26 CFR 31.3121(d)-2: (d)-2 Who are employers.

(a) Every person is an employer if he employs one or more employees. Neither the number of employees employed nor the period during which any such employee is employed is material for the purpose of determining whether the person for whom the services are performed is an employer.

(b) An employer may be an individual, a corporation, a partnership, a trust, an estate, a joint-stock company, an association, or a syndicate, group, pool, joint venture, or other unincorporated organization, group, or entity. A trust or estate, rather than the fiduciary acting for on behalf of the trust or estate, is generally the employer.

(c) Although a person may be an employer under this section, services performed in his employ may be of such a nature, or performed under such circumstances, as not to constitute employment (see § 31.3121(b)-3).