What term is used to describe the percentage of alcohol in beverages, where 90 proof indicates a specific percentage?

The term that refers to the percentage of alcohol in beverages, where 90 proof indicates a specific percentage, is proof. Proof is a measure of the strength of an alcoholic beverage that is calculated as twice the percentage of alcohol by volume (ABV). For example, a beverage that is 90 proof contains 45% alcohol by volume. This measurement system allows consumers to understand the potency of different alcoholic drinks. In essence, the proof indicates the alcohol content in a way that is easily recognizable and widely used, particularly in the United States.

While terms like alcohol by volume provide specific percentages of alcohol in a drink, proof is the traditional measure that equates to a specific volume of alcohol content, making it the most relevant answer in this context. Concentration and alcohol level also relate to alcohol content but do not specifically refer to the traditional proof system.