Alcohol By Volume is that usually single digit and sometimes double digit percentage number often including a decimal point. ABV is a standard worldwide measure of how much alcohol is contained and is expressed as a percentage of total volume. In the case of IPA, that percentage is most often higher than other brew styles.
To determine the ABV of a given beer, a brewer typically uses what's called a hydrometer, an instrument measures the density of liquid in relation to water. The hydrometer will be calibrated to read 1.000 in water at 60°F, and the denser the liquid (example: add sugar to the liquid), the higher the hydrometer reading.
Before yeast cells are introduced to ferment beer, the liquid is called "wort (pronounced wert)," containging various sugars that were previously extracted from grain. A brewer will take a hydrometer measurement of the wort (at 60°F) to determine what's called the original gravity (OG). Then yeast is added to the wort, and fermentation begins. As the yeast cells eat the sugar in the wort, two wonderful by-products result: carbonation (CO2) and alcohol. And once the brewer has determined that the hungry yeast has had enough (days, weeks or months), an additional hydrometer reading (at 60°F) will record what's called the final gravity (FG).
Since both measurments were taken at 60°F, a fairly simple calculation will deliver the ABV
(OG-FG) x 131 = ABV