In the United States, 100-proof alcohol means that the liquor is 50% alcohol by volume. Though alcohol by volume remains the same regardless of country, the way different countries measure proof varies.
Starting in the 16th century, the strength of an alcohol was proven by using it to douse some gunpowder and then attempting to ignite the gunpowder. If the alcohol was weaker than 57.15% alcohol by volume, the gunpowder would not ignite. Because of this, the point at which the gunpowder begins to ignite became known as 100-proof. In recent times, however, the United States defined their version of the proof system as double the percentage of alcohol in a beverage at 60 degrees Fahrenheit. The original proof system remains in use in the United Kingdom.