an incident involving incorrect presence of a vehicle, person or another aircraft on the runway.

An example of a runway incursion is when an airport vehicle such as a runway inspection truck crosses the active runway without clearance, or when an aircraft lines up out of sequence.  In most cases nothing bad will happen, except maybe cause a go-around that only costs time and fuel.  But many serious accidents have happened due to runway incursions, and the chance of an accident is great in poor weather and visibility conditions such as fog.

In the 1972 Chicago-O’Hare runway collision, North Central Airlines Flight 575 (a McDonnell Douglas DC-9) collided during its takeoff roll with Delta Air Lines Flight 954 (a Convair CV-880) while the CV-880 was taxiing across a fog-shrouded runway at O’Hare International Airport in Chicago, Illinois, killing 10 people and injuring 17.  The tenerife disaster was another notable accident, killing more than 500 people.