What is the meaning of proof in whiskey?

The proof of a distilled alcoholic beverage is a measure of its alcohol content, expressed as the number of U.S. proof gallons (or in certain countries, UK proof gallons) of pure alcohol in a 1 US liquid gallon (or Imperial gallon) of liquid. Because the specific gravity of ethyl alcohol is 0.7939 at 68 °F (20 °C), the volume fraction of absolute ethanol in a proof spirit at 68 °F is half its proof strength, with the other half being water.

For example, a spirit that is 100 proof has 50% alcohol by volume (ABV). In the US, whiskey must be bottled at a proof of at least 80 (40% ABV) to qualify for the designation of "straight" whiskey, and most bourbon whiskeys are bottled at 90–115 proof. Whiskey cannot be legally bottled at a proof higher than 125 in the US.

The proof of a whiskey can affect its flavor and口感. Higher-proof whiskeys tend to be more intense and flavorful, while lower-proof whiskeys are smoother and more mellow.