What is the system of measure and expressing alcoholic content?

Proof is a system of measuring the alcoholic content of a beverage. The proof of a distilled spirit is twice its alcohol by volume (ABV). For example, a spirit that is 50% ABV is 100 proof.

Alcohol by volume (ABV) is the amount of alcohol present in a beverage by volume. It is expressed as a percentage. For example, a beer that is 5% ABV contains 5% alcohol by volume.

Volume per volume (v/v) is a measure of the amount of alcohol present in a beverage by volume compared to the total volume of the beverage. It is expressed as a ratio. For example, a drink that is 2:1 v/v contains two parts alcohol to one part non-alcoholic liquid.

Alcohol by weight (ABW) is the amount of alcohol present in a beverage by weight. It is expressed as a percentage. For example, a wine that is 12% ABW contains 12% alcohol by weight.

Weight per volume (w/v) is a measure of the amount of alcohol present in a beverage by weight compared to the total weight of the beverage. It is expressed as a ratio. For example, a drink that is 10:1 w/v contains ten grams of alcohol to one kilogram of non-alcoholic liquid.