In order to legally call a whiskey “bourbon”, it must meet a small but important set of criteria. First, it must be made in the United States. Next, it must be made from a mixture or at least 51% corn (the fermenting “mash” can be up to 100% corn, legally). It must be aged in newly-made oak wood barrels that have been briefly burned — “charred” — on the inside. No flavoring or coloring of any kind can be added, and it must be aged for a minimum of four years.
Lastly, it can be distilled to no more than 80% alcohol (160 proof), it cannot be put in barrels at more than 62.5% alcohol (125 proof) and it can’t be bottled at less than 40% alcohol (80 proof). These last couple of points are a bit esoteric to all of us non-distiller types, but they’re important for reasons I shall explain.