A measure of alcohol concentration in a beverage.
Proof is equivalent to twice the percentage of alcohol.
For instance, 80 proof distilled spirits contain 40% alcohol.
Originating during the 18th century, when payments to sailors included rations of brandy.
To ensure that the brandy being used as payment has not been watered down or was of good quality, it was proved by dousing gunpowder in a sample of brandy, and testing to see if it would ignite.
If it didn't ignite, the solution had too much water in it and the proof was considered low or "underproof".
If it did ignite that was considered "proof" of sufficient alcohol content.