Mike, this is from Wikipedia:
In the definition current in the United States, the proof number is twice the percentage of the alcohol content measured by volume at a temperature of 60 Â°F (15.5 Â°C). Therefore "80 proof" is 40% alcohol by volume (most of the other 60% is water), and pure alcohol would be "200 proof". If a 150-proof beverage is mixed half-and-half with water, the product is 75 proof. US proof numbers are properly cited as, for instance, "86 proof," not "86 degrees proof." The use of the word "degrees" in this context is incorrect.
US Federal regulation (CFR 27 5.37 Alcohol Content) requires that liquor labels state the percentage alcohol by volume (sometimes abbreviated ABV). The regulations permit (but do not require) a statement of the proof as long as it is right next to the percentage alcohol by volume.