A runway incursion is an incident where an unauthorized aircraft, vehicle or person is on a runway. This adversely affects runway safety, as it creates the risk that an airplane taking off or landing will collide with the object. It is defined by the International Civil Aviation Organization (ICAO) on April 27, 2006, as:
Any occurrence at an aerodrome involving the incorrect presence of an aircraft, vehicle, or person on the protected area of a surface designated for the landing and take-off of aircraft.
In October 2007, the Federal Aviation Administration dropped its own definition of "runway incursion" and adopted the above. The difference between the two was "that ICAO defines a runway incursion as any unauthorized intrusion onto a runway, regardless of whether or not an aircraft presents a potential conflict. For the FAA, an incident without an aircraft in potential conflict– such as an unauthorized aircraft crossing an empty runway– was defined as a 'surface incident' and not a runway incursion."
(source Wikipedia)