What is the common definition of proof in alcoholic drinks?

Proof is a standard measurement used to indicate the alcohol content in a beverage, and it is defined as twice the percentage of alcohol by volume (ABV). For example, if a spirit has an ABV of 40%, its proof would be 80. This system of measuring alcohol content is significant not only for legal regulations but also for providing consumers with a clear understanding of potency. Knowing the proof helps bartenders and consumers alike gauge the strength of the drink and make informed choices regarding consumption and mixing.

In this context, other options do not align with the standard definition of proof. Sugar content (first option) and taste quality (third option) do not relate to the measurement of alcohol strength, while the speed of mixing (fourth option) pertains more to the bartending technique rather than the characteristics of the drink itself.