CFR /  Title 20  /  Part 404  /  Sec. 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.