What is the key characteristic of a "sour" cocktail?

A "sour" cocktail is defined primarily by the inclusion of a citrus element, which is usually lemon or lime juice. This ingredient is essential as it provides the tangy flavor that characterizes sour cocktails, balancing the sweetness of the spirits and other components. The acidity from the citrus not only adds brightness to the drink but also enhances the overall flavor profile, making it refreshing and vibrant.

The classic examples, such as the Whiskey Sour or the Pisco Sour, showcase how the balance between sourness and sweetness leads to a harmonious blend. Citrus juice plays a crucial role in achieving the distinctive taste that defines this category of cocktails.

Other options, while they pertain to certain types of cocktails, do not encapsulate what makes a sour cocktail unique. For instance, the presence of bitters is typically associated with different styles of mixed drinks rather than being a defining feature of sour cocktails. Similarly, serving methods like blending or lacking mixer elements do not specifically relate to the sour classification but rather apply to various cocktails across different styles.