"100 proof" refers to the strength of an alcoholic beverage. In the United States, the proof system measures the alcohol content of spirits. The proof number is double the percentage of alcohol by volume (ABV). So, 100 proof means the beverage is 50% alcohol by volume. This system originated in the 18th century when alcohol was tested for its strength by mixing it with gunpowder and seeing if it would ignite. If it did, it was considered "proof" that the alcohol was strong enough.

In many other countries, ABV is used directly, without the proof system.