What is 90 proof alcohol?

90 proof alcohol means that the alcoholic beverage contains 45% alcohol by volume (ABV).

Here's how it works:

* Proof is a measure of the alcohol content in a beverage, specifically the double of the ABV.

* So, a 90 proof spirit is 90/2 = 45% alcohol by volume.

This means that in a 100ml bottle of 90 proof alcohol, 45ml of the contents would be pure alcohol.