20 CFR 404.1003: Employment.

Employment means, generally, any service covered by social security performed by an employee for his or her employer. The rules on who is an employee and who is an employer are contained in §§ 404.1005 through 404.1010. Section 404.1004 states the general rule on the kinds of work covered as employment. Exceptions to the general rule are contained in §§ 404.1012 through 404.1038 which explain the kinds of work excluded from employment. All of these rules apply to current work unless otherwise indicated.