Bourbon (H/T Caskers):
Federal regulations in the United States (27 C.F.R. 5 to be specific) require that bourbon be made from a mashbill of at least 51% corn before being matured in new oak barrels and bottled at 80 proof or higher. Bourbon that meets these requirements and that has been aged for a minimum of at least two years may be called straight bourbon.
The Whiskey Post 