The Federal Standards of Identity for Distilled Spirits state that bourbon made for U.S. consumption must be:
Produced in the United States
Made from a grain mixture that is at least 51% corn
Aged in new, charred oak containers
Distilled to no more than 160 (U.S.) proof (80% alcohol by volume)
Entered into the barrel for aging at no more than 125 proof (62.5% alcohol by volume)
Bottled (like other whiskeys) at 80 proof or more (40% alcohol by volume)
Bourbon has no minimum specified duration for its aging period. Products aged for as little as three months are sold as bourbon. The exception is straight bourbon, which has a minimum aging requirement of two years. In addition, any bourbon aged less than four years must include an age statement on its label.
Bourbon that meets the above requirements, has been aged for a minimum of two years, and does not have added coloring, flavoring, or other spirits may (but is not required to) be called straight bourbon.