Entrance of an authorized or unauthorized individual into a secure area through an entry, without the use of an access card, by walking or driving immediately behind an authorized individual who has used a valid access card to gain entry and before the portal has been allowed to close. See Access Control.
Tailgating is the practice of driving on a road too closely behind another vehicle, such as less than the travel distance in two seconds or equivalently, one vehicle-length for every 5 mph of the current speed. It is generally considered to be impolite behaviour, and may also be illegal in some jurisdictions due to its increased potential for causing a car accident.