A device used to the measure the specific gravity of a liquid.
By measuring the specific gravity before fermentation (original gravity or OG) and after fermentation (final gravity or FG) one can estimate the alcohol concentration of the final product.
The difference between the OG and FG can be used as an estimate of the amount of sugars converted to alcohol. This can then be use to calculate the alcohol by volume:
%ABV = 132.715*(OG - FG)