Just like Sherry can't be called Sherry or Scotch can't be called Scotch unless they meet certain geographic, constituent, and/or process requirements, Bourbon has these (Congress 1964):
- Mash bill must be at least 51% corn. The rest is often wheat ("wheated bourbon", the lightest and sweetest), rye, barley.
- Must enter barrels at <=125 proof. Must exit barrels at <=160 proof (thus you get a standard "cask strength" maximum 80% abv, some really great stuff in this category).
- Aged in new charred oak barrels. This creates a constant need for new barrels by distillers, and a secondary market for other spirits to age in (bourbon barrel beer, bourbon barrel wine, etc.) - flavors and alcohol seep back out of the barrel.
- All the above must be done within the United States.
Age Statements:
- If aged a minimum of 2 years, it can be called Straight Bourbon (not required to be labeled thusly).
- If aged less than 2 years, age statement must be on bottle.
- Age statement must be that of the youngest constituent bourbon (some distillers blend a variety of ages; most blend a variety of barrels from different locations in their rickhouse - different characters).
Bonus: "Bottled in Bond" / "Bonded" Whiskeys (not just Bourbons):
- Distilled entirely by 1 distiller at 1 location in the United States (i.e., the label meant something).
- Aged a minimum of 4 years at a facility under federal supervision(!) (not doctored with iodine, colorants, turpentine).
- Entered bottle at >=100 proof (not watered down).
The Bruery specializes in those beers. They end up with some delicious heavy hitters like Black Tuesday, which can run 17%-19%