There is a complete definition you can find at Wikipedia, but this is the part I found the most interesting:
In the 18th century and until 1 January 1980, Britain defined alcohol content in terms of “proof spirit,” which was defined as the most dilute spirit that would sustain combustion of gunpowder. The term originated in the 18th century, when payments to British sailors included rations of rum. To ensure that the rum had not been watered down, it was “proofed” by dousing gunpowder in it, then testing to see if the gunpowder would ignite. If it did not burn, the rum contained too much water and was considered to be “under proof.” A proven sample of rum was defined to be 100 degrees proof; this was later found to occur at 57.15% alcohol by volume, which is very close to a 4:7 ratio of alcohol to total amount of liquid. Thus, the definition amounted to declaring that (4÷7) × 175 = 100 degrees proof spirit.
From this it followed that pure, 100% alcohol had (7÷7) × 175 = 175 degrees proof spirit, and that 50% ABV had (3.5÷7) × 175 = 87.5 degrees proof spirit. To convert percentage of alcohol by volume to degrees proof spirit, multiply the percentage by 1.75.
In the United Kingdom, the proof-to-ABV ratio is 7:4. In the United States, it is 2:1. By these standards it takes less drinks to get your buzz in the United Kingdom.