Wikipedia: Blood alcohol content

Blood Alcohol Content, often abbreviated BAC, is the percentage of blood by volume that is alcohol. It is used as an objective (and ideally therefore, legally uncontestable) measure of the level of impairment of an individual, especially as it pertains to the operation of a [motor vehicle]?.

In the United States, most states have a legislated BAC that defines inebriation? at a specific percentage (often about 0.10 %). The federal government is forcing a change to 0.08%. In some states, underage drunken drivers are considered legally impaired at lower levels (perhaps 0.02%).

In Australia, the limit is 0.05% in most states and territories, and either 0.02% or zero for inexperienced drivers (learner drivers or those on probationary licenses). It is enforced by [random breath testing]?.