Which ingredient is NOT used in the preparation of an Old-Fashioned cocktail?

The Old-Fashioned cocktail is traditionally made with a base spirit, usually bourbon or rye whiskey, combined with a few key ingredients that define its classic preparation. Ice is used to chill the drink, bitters add depth and complexity of flavor, and sugar serves as a sweetener to balance the strong flavors of the whiskey and bitters.

Rum, while a popular spirit in many cocktails, is not a traditional ingredient in an Old-Fashioned. This cocktail specifically highlights whiskey, making it distinct from other drinks that may utilize rum or other spirits. Thus, the ingredient that is not used in the preparation of an Old-Fashioned is indeed rum.