A breathalyzer (or breathalyser) is a device used by police forces to detect the amount of alcohol in one's breath during traffic stops.
Although there are supposedly ways of 'fooling' the breathalyzer (that is, making one's blood alcohol content appear lower) an episode of the Discovery Channel's Mythbusters proved that the substances usually recommended in this practice -- such as breath mints, mouthwash, and onion -- are ineffective.