What ingredient differentiates the Old Fashioned cocktail from other whiskey cocktails?

The ingredient that notably differentiates the Old Fashioned cocktail from other whiskey cocktails is bitters. While brown sugar is sometimes used in variations of an Old Fashioned, the classic recipe typically calls for a sugar cube or simple syrup, making bitters the truly defining element.

Bitters add complexity and depth to the drink, enhancing the whiskey's flavor while balancing the sweetness. The Old Fashioned is known for its simplicity, and bitters serve as the essential component that elevates its profile, distinguishing it from other whiskey cocktails that may rely more heavily on ingredients like vermouth or mixers.

This unique characteristic of using bitters is what firmly establishes the Old Fashioned's identity in the cocktail world, making it a classic and fundamentally different option compared to other whiskey-based drinks.