What does 'proof' measure in distilled spirits?

Proof measures the alcohol content in distilled spirits and is defined as twice the percentage of alcohol by volume (ABV). For instance, if a spirit has an ABV of 40%, it is considered to be 80 proof. This standard provides a straightforward way to quantify the strength of the alcohol contained in the beverage, allowing consumers to compare the potency of different products easily.

While the other options address various aspects of spirits, they do not relate directly to the measurement of proof. Acidity level pertains to the balance of acids present, caloric content refers to the energy value of the drink, and age typically indicates how long the spirit has been aged in barrels, which does not affect proof directly. Understanding proof is essential when determining how strong a drink is and in making informed choices about consumption.