Proof

Proof is an old measure of alcohol content used mainly in the USA and Britain. In the USA the value is twice the alcohol content in percent by volume: 100 proof equals 50 % vol.

The term is said to go back to an earlier test in which one checked whether a spirit mixed with gunpowder would still burn. The British proof system calculated differently, 100 British proof being roughly 57 % vol. Today only % vol. is used in Europe, while American bottles often state both.