Bourbon is an American form of whiskey, made from at least 50% corn (though more typically 70%, with the remainder being wheat, rye, and other grains), distilled to no more than 160 proof, and aged in new charred white oak barrels for at least two years (usually much longer). It is then adjusted to 80-100 proof and bottled. The name derives from Bourbon County, Kentucky, which was itself named after the French royal family at the time of the American revolution. Stories about its origins there are doubtful. In particular, its invention is often credited to Baptist minister and distiller Elija Craig, whose distillery is located in what is today Bourbon county, but which was not in that county at the time of its supposed invention. Today the spirit is made primarily in Kentucky, Tennessee, and Virginia.
A further refinement introduced by Scottish chemist Dr. James C. Crow was the "sour mash" process, by which each new fermentation is conditioned with some amount of spent beer (previously fermented mash that has been separated from its alcohol), much the way sourdough bread is made from starter.