Because the definition is a regulation rather than a custom, the questions people argue about mostly have flat answers.
The rules in plain terms
At least 51% corn in the mash bill, with the remainder usually rye or wheat plus malted barley. New charred oak, always — and the word "new" is what most sharply separates bourbon from Scotch, which reuses casks as a matter of course. Nothing may be added but water.
Straight bourbon
A further standard: at least two years in the wood, and if it is under four years old the age must be stated on the label. This is why so many bourbons carry either a four-year statement or none at all.
Does it have to come from Kentucky?
No. Anywhere in the United States. Kentucky's association with bourbon is historical and commercial rather than legal, and bourbon is now made in most states.
Bottled-in-bond
An older and stricter standard from 1897: the product of one distilling season at one distillery by one distiller, aged at least four years in a federally supervised warehouse, and bottled at exactly 100 proof. It predates modern labelling law and was originally a guarantee of authenticity rather than of quality.