Proof is a measurement of alcohol content in a liquid by volume. Proof readings vary in different countries. In the United States, 200 proof corresponds to 100% alcohol, while 100 proof corresponds to 50% alcohol. In the USA, proof is twice the ABV. In Britain, by contrast, 100 proof corresponds to 57.15% alcohol.
The proof scale took its root in the days before lab instruments were commonly available. To "prove" the alcohol content of a beverage, a trader would wet gunpowder with it. If the powder still lit, the beverage was "proven" to contain a minimum amount of ethanol. A solution with that minimum amount was assigned 100 proof degrees, giving rise to the proof scale. Once more precise measures of alcohol content were developed, the ABV of this minimum amount was determined to be 57.15% - a figure that still determines the proof scale in Britain and elsewhere. Once this method of proof was superseded by lab equipment, though, America modified its scale to assign 100 degrees of proof to a solution with 50% ABV, for more convenient figures.