BAC is the blood-alcohol content in a person's blood. It is expressed as a percentage. There is a presumption in every state that a BAC of .08 or higher is per se proof of impaired driving or DUI for driving a car on a public road.
For individuals under the age of 21 in Tennessee the BAC presumption is .02 or higher for per se proof of impairment. For commercial licenses a BAC of .04 or higher is proof of impairment.
Blood-alcohol concentration (BAC) is expressed in percent weight by volume (% w/v) based upon grams of alcohol per 100 cubic centemeters of blood or per 210 L of breath. A BAC of 0.08 % w/v means 0.08 g of alcohol per 100 cm³ of blood (0.008g/100cc) or 0.08 g of alcohol per 210 L of breath.
Incidentally the BAC levels for some other countries is: China .02, India .03, Norway .02, England .08, France .05.