Well, it is now. At least in the US.
Proof is much more interesting than that, though (apologies if I’m preaching to the choir).
The term originated in the 16th century, when payments to British sailors included rations of rum. To ensure that the rum had not been watered down, it was “proved” by dousing gunpowder with it and then testing to see whether the gunpowder would ignite. If it did not, then the rum contained too much water and was considered to be “under proof”. Gunpowder would not burn in rum that contained less than 57.15% ABV. Therefore, rum that contained this percentage of alcohol was defined to have “100° (one hundred degrees) proof”
Alcohol is much better than website adverts.
