Blood alcohol content (BAC) is the amount of alcohol in the bloodstream. If someone were to have a BAC of .10, this would mean that one-tenth of 1% of the blood in their body is alcohol. The more alcohol a person consumes the more intoxicated and impaired they become. The alcohol percentage is determined by the amount of alcohol consumed, the time in which the alcohol was consumed, and body weight. Eating prior to, or while drinking, may also temporarily influence BAC, reducing its effects.
Here is a BAC calculator.