ABV is short for Alcohol by Volume. In spirits, wine, beer, etc. it’s the percentage of the drink that’s pure alcohol.
Proof is 2X ABV, so a drink that’s 50% ABV like Bottled in Bond Whisky, is 100 proof. In the 18th century, the Royal Navy would douse gunpowder in rum from their run rations. If the gunpowder ignited, it was proof that it was sufficiently alcoholic. If it didn’t… MUTINY!!!