What does "proof" mean when looking at alcohol?

Question

For example, 80 proof Vodka. What does 80 proof mean?

Answer 1

The proof number (in the US) is twice the percentage of alcohol (ethanol) by volume, so 80 proof vodka is 40% alcohol by volume. The history of why it's called 'proof' is somewhat involved.

https://en.wikipedia.org/wiki/Alcohol_proof

Comments

And what's scary is you didn't have to look that up. 

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Well, um... I didn't have to look up the proof number being twice the alcohol percentage, but the old gunpowder test to determine the alcoholic strength for tax purposes was new to me. (hic... 'scuse me!) 

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