Alcoholic proof
Stronger liquors are distilled after fermentation is complete to separate the alcoholic liquid from the remains of the grain, fruit, or whatever it was made from. The idea of distillation is that a mixture of liquids is heated, the one with the lowest boiling point will evaporate (or "boil off") first, and then the one with the next lowest boiling point, and so on. The catch is that water and alcohol form a mixture (called an azeotrope) that has a lower boiling point than either one of them, so what distills off first is that mixture of 95% alcohol and 5% water.  Thus a distilled liquor cannot be stronger than 95% (190 proof); there are other techniques for separating liquids that can produce 100% ethanol (or "absolute alcohol"), but they are used only for scientific or industrial purposes. 100% ethanol does not stay 100% for very long, because it is hygroscopic and absorbs water out of the atmosphere.