Alcoholmeter / Spirit Hydrometer
Dip this in your hypothetical pure spirit and it will tell you the ABV directly.
Good for making sure the supermarket isn't ripping you off with their own-brand vodka!
This hydrometer will not work for beer or wine. You need a beer and wine hydrometer for that.
This one works because spirits contain only water and alcohols so the measured density can give a direct result due to the lack of any complications like sugars or other solubles.
No hydrometer can tell you percentage of beer or wine or cider directly. Instead, you must measure the sugar density at the beginning and end of fermentation and extrapolate the ABV from the difference between these. Please look for the other hydrometer types if this is what you need.