20 C.F.R. § 404.1066

Trade or business in general.

            For you to be covered as a self-employed person for social security purposes, you must be engaged in a trade or business. You can carry on a trade or business as an individual or as a member of a partnership. With some exceptions, the term
            trade or business
             has the same meaning as it does when used in section 162 of the Code.

Official source