To be legally sold as bourbon, a whiskey's mash must contain at least 51% corn. The Federal Standards of Identity for Distilled Spirits in the United States define bourbon as a type of whiskey that is produced from a grain mixture (mash) that is at least 51% corn. The remaining grains used in the mash can include malted barley, rye, or wheat.
The use of corn as the primary grain in bourbon contributes to its distinctive flavor profile. It often imparts a sweet and rich character to the whiskey. Additionally, bourbon must meet other criteria, such as being aged in new charred oak barrels, distilled to no more than 160 proof (80% alcohol by volume), and entered into the barrel for aging at no more than 125 proof (62.5% alcohol by volume).
It's worth noting that these regulations are specific to the United States and may vary in other countries that produce whiskey.