The Small Business Act defines a small business as one that
is organized for profit;
has a place of business in the United States;
operates primarily within the United States or makes a significant contribution to the U.S. economy through payment of taxes or use of American products, materials, or labor;
is independently owned and operated; and
is not dominant in its field on a national basis.
The business may be a sole proprietorship, partnership, corporation, or any other legal form.
Source: An Overview of Small Business Contracting