The "bona fide use of a mark in the ordinary course of trade and not made merely to reserve a right in a mark." 15 U.S.C. §1127.
For federal registration purposes, using a mark in "interstate commerce" is required. Whereas "use in commerce" refers merely to commercial use in trade, "interstate commerce" occurs only when the commercial use in trade affects, either directly or indirectly, commerce that occurs across state lines. See the interstate commerce glossary entry for more information about the requirements of interstate commerce.
Interstate commerce is required whether applying for a trademark under an actual use application, filing an Intent to Use application, or filing a renewal application. With an Intent to Use application, however, you don't have to demonstrate actual use until sometime within six months after publication (or later if you file an extension).
State and common law marks are protected even when there is no interstate commerce.