A term used to indicate the amount of alcohol in liquor or other spirits. In the United States, proof is exactly twice the percentage of alcohol. Therefore, a bottle of liquor labeled 86 Proof contains 43 percent alcohol. Proof v. To dissolve yeast in a warm liquid (sometimes with a small amount of sugar) and set it aside in a warm place for 5 to 10 minutes until it swells and becomes bubbly. This technique proves that the yeast is alive and active and therefore capable of leavening a bread or other baked good.