If you're a whiskey (and especially bourbon) drinker, you need to know what that means. By definition, whiskey (or whisky, in Scotland) is a spirit distilled from fermented grain mash — grain varieties include wheat, rye, barley, and corn — and then aged in wooden barrels. Whiskey is made all over the world and there are many popular styles including Scotch whisky, Irish whiskey, and American whiskey.