(n.) A unit of measure for alcohol strength calculated in the United States by doubling the percentage of alcohol by volume (ABV). For example, a spirit made of 40 percent alcohol is considered to be 80 proof.
The term is thought to originate from the 18th-century English practice of testing rum’s strength by igniting liquor-soaked gunpowder to see if the alcohol was strong enough to catch fire. This experiment would only work if the rum was over 57.15% ABV, a level that was then termed 100 proof. Though the United States used a simplified version of this formula of doubling the percentage of alcohol to determine the proof, the English used this correlating ratio until 1980, when they began labeling liquor in terms of ABV. In the United States, the term is not required on bottle labels, though it is permitted.