Short for "blood alcohol concentration." BAC talks about the amount of alcohol in your bloodstream and is measured in percentages. BAC can be measured either by breath, blood or urine testing and is often used by law enforcement to decide whether a motorist is "legally drunk." Most states have adopted BAC laws that make it illegal to drive with a BAC at or above a set amount. Most states have adopted 0.08% as the BAC limit state-wide.