A measure of alcoholic strength expressed as an integer twice the percentage of alcohol present (by volume) WordNet 3.0 & Open English WordNet

Alcohol proof is a measure of the content of ethanol in an alcoholic beverage. Wikipedia

A measure of an alcoholic drink's strength Wikipedia Disambiguation

Measure of the content of ethanol (alcohol) in an alcoholic beverage Wikidata

A measure of the alcohol content of liquor. Originally, in Britain, 100 proof was defined as 57.1% by volume (no longer used). In the US, 100 proof means that the alcohol content is 50% of the total volume of the liquid; thus, absolute alcohol would be 200 proof. Wiktionary