Bourbon is an American form of whiskey made from (pursuant to U.S. trade law) at least 51% corn, or maize, (typically about 70%) with the remainder being wheat, rye, and malted barley. It is distilled to no more than 160 (U.S.) proof, and aged in new charred white oak barrels for at least two years — or perhaps longer. It must be put into the barrels at no more than 125 U.S. proof; in this way it is similar to Scotch whisky, which is also aged in charred barrels. Most of the time it is then adjusted to 80–100 proof and bottled, although some are bottled at “cask strength”.
Johnson's (revised) Universal Cyclopaedia, 1836
Twelve Days in the Tombs, by Jonathan Harrington Green, 1851
"After this the old man turned upon his heel, and called the barkeeper to give him a glass of old " Bourbon whiskey," and to send a quart down to his men..."
Life and Liberty in America, by Charles Mackay, 1859
"A thoughtful friend at Cincinnati had given us on starting a bottle of Bourbon whiskey twenty years old"
Dictionary of Americanisms, by John Russell Bartlett, 1860
"BOURBON. Whiskey from Bourbon county, Kentucky. A term generally used to distinguish the better kinds of whiskey, which are mostly made from corn instead of rye."
What we eat, by Thomas H. Hoskins, 1861
"It is known as Bourbon whiskey, from the county of Bourbon, one of the oldest counties of Kentucky, where formerly considerable quantities of whiskey were made."
History of Bourbon Whiskey
Bourbon can legally be made anywhere in the United States. However, all but a few of these brands are made in Kentucky, and the drink is associated strongly with that state. When American pioneers pushed west of the Allegheny Mountains following the Revolution, the first counties they founded covered vast regions. One of these original, huge counties was Bourbon, established in 1785 and named after the French royal family. While this vast county was being carved into many smaller ones, early in the 19th century, many people continued to call the region 'Old Bourbon.' Located within 'Old Bourbon' was the principal Ohio River port from which whiskey and other products were shipped to market. 'Old Bourbon' was stenciled on the barrels to indicate their port of origin. 'Old Bourbon' whiskey was different because it was the first corn whiskey most people had ever tasted, and they liked it. In time, 'bourbon' became the name for any corn-based whiskey."
A refinement introduced by Dr. James C. Crow was the sour mash process, by which each new fermentation is conditioned with some amount of spent "Feed Mash" (previously fermented mash that has been separated from its alcohol), in much the same way that sourdough bread is made from starter. The acid introduced by using the sour mash controls the growth of bacteria that could taint the whiskey. As of 2005, all straight bourbons use a sour mash process. Crow developed this refinement while working at the Old Oscar Pepper Distillery (now the Woodford Reserve Distillery) in Woodford County, Kentucky. (Feed Mash is so named because these spent grains are also used as an ingredient in animal feed.)
Most bourbons are distilled in Kentucky, and it is sometimes said that only Kentucky whiskey can properly be called bourbon; this is, however, not true, as a few exceptions to the rule, such as Virginia Gentleman, demonstrate. As of today, there are no running distilleries within the current boundaries of Bourbon County due to new counties being formed from Bourbon County since early whiskey making days.
An act of the U.S. Congress in 1964 declared bourbon to be "America's Native Spirit" and its official distilled spirit.  A concurrent resolution of the U.S. Congress restricted bourbon to U.S. production. Some stories about its origins therein may or may not be accurate, such as its invention by Baptist minister and distiller Elijah Craig.
Drinks made with Bourbon Whiskey