Bourbon

Bourbon is an American form of whiskey named for Bourbon County, Kentucky. By United States law, it consists of at least 51% corn - typically about 70% - with the remainder being wheat and/or rye, and malted barley. It is distilled to no more than 160 (U.S.) proof, and aged in new charred oak barrels for at least two years.

The two years maturation process is not a legal requirement for a whiskey to be called "bourbon," but it is a legal requirement for "straight bourbon." However, in practice, most bourbon whiskeys are aged for at least four years.

Source: Wikipedia