Alcohol by volume (abbreviated as abv or ABV) is a standard measure of how much alcohol (ethanol) is contained in an alcoholic beverage (expressed as a percentage of total volume). The abv standard is used worldwide.
In some countries, alcohol by volume is referred to as degrees Gay-Lussac (after the French chemist Joseph Louis Gay-Lussac).
Another way of specifying the amount of alcohol is alcoholic proof, which is twice the alcohol-by-volume number.
In the United States, a few states regulate and tax alcoholic beverages according to alcohol by weight (ABW), expressed as a percentage of total mass. Some brewers print the ABW (rather than the ABV) on beer containers, particularly on low-point versions of popular domestic beer brands.
At relatively low ABV, the alcohol percentage by weight is about 4/5 of the ABV (e.g., 3.2% ABW is equivalent to 4.0% ABV). However, because of the miscibility of alcohol and water, the conversion factor is not constant but rather depends upon the concentration of alcohol. 100% ABW, of course, is equivalent to 100% ABV.