Alcoholic proof
In the definition of the United States, the proof number is twice the percentage of the alcohol content measured by volume at a temperature of 60°F (15.5°C). Therefore, "80 proof" is 40% alcohol by volume (most of the other 60% is water). If a 150 proof beverage is mixed half-and-half with water, the drink is approximately 75 proof (actual proof in this example would be 76.28 due to the nature of ethanol/water solutions).