Whiskey (Canadians and Scots spell it “whisky” and the Irish spell it “whiskey”- most U.S. dictionaries prefer the Irish spelling) is distilled alcohol made from a fermented mash of grain, matured in oak containers (except that corn whiskey doesn’t have to be so stored) and bottled at not less than 80 proof.
Bourbon is a subcategory of whiskey. To qualify as bourbon- the law by international agreement stipulates:
It must be made in USA.
It must be made from a mash containing at least 51% and no more than 79% Indian corn.
It must be aged for at least 2 years- although most bourbons are aged for 4 or more years.
The barrels can be made of any kind of new oak, charred on the inside. (Now all distillers use American White Oak. It is porous enough to help bourbon age well, but not so porous that it will allow barrels to leak.)
It must be distilled at no more than 160 proof (80% alcohol by volume).
Nothing can be added at bottling to enhance flavor or sweetness or alter color.
Other grains used to make bourbon, though not stipulated by law, are malted barley and either rye or wheat. Bourbon can be made anywhere in the USA but most are in Kentucky. The name, Bourbon, comes from Bourbon County in the central bluegrass region of Kentucky. (Only the state of Kentucky can produce bourbon with its name on the label).There are no distillers in Bourbon County, Kentucky right now.
Tennessee whiskey is very similar to Bourbon. It need only be comprised of 51% of any grain, with corn being the most often used. The distinct difference in Tennessee Whiskey is due to a filtration process where the whiskey is allowed to slowly drip through 10 feet of sugar-maple charcoal, a process that can take up to 2 weeks for one batch. The whiskey is then transferred to a charred barrel for aging.
International laws require anything bearing the label “Scotch” to either be distilled in Scotland or matured for a minimum of 3 years in oak casks or distilled elsewhere meeting the same standards. Scotch is usually distilled twice, sometimes 3 times. Scotch is made from Malt and or Grain whiskeys. Malt is whisky made entirely from malted barley. Grain is whiskey made from malted and unmalted barley along with other grains
Single malt is produced by a single distillery. Unless the whisky is described as “single-cask”, it will contain the same type of malt yet from many casks.
Vatted malt is created by combining malt whiskies produced by different distilleries. It is usually labeled as “pure malt” or “malt”
Blended whiskies, usually cheaper and made from a mixture of malt and grain from many distilleries, are blended to produce a flavor consistent with a brand that’s independent of a particular distillery.
Whiskies mature only in the cask and not the bottle. So, if you have an old bottle of whiskey, scotch or bourbon, it may be rare but the aging process stopped when it was bottled so it won’t be any better than when it was first bottled.