If a liquor is 100 proof, it means that it contains 50% alcohol by volume (ABV). The term "proof" is a historical measurement that was used to determine the alcohol content of spirits. In the United States, proof is defined as twice the percentage of alcohol by volume. Therefore, if a liquor is labeled as 100 proof, it contains 50% alcohol. Similarly, if a liquor is labeled as 80 proof, it contains 40% alcohol by volume. It's important to note that proof is primarily used in the United States, while many other countries express alcohol content solely in terms of ABV.