What is a Runway Incursion?
 

The FAA defines a Runway Incursion as: 

 "Any occurrence at an airport involving an aircraft, vehicle, person, or object on the ground that creates a collision hazard or results in loss of separation with an aircraft taking off, intending to take off, landing, or intending to land." 

Runway incursions are made up of pilot deviations, operational errors, and vehicle or pedestrian deviations. 

Glossary

pilot deviation - The pilot makes a manuever, either planned or unplanned in the eyes of a controller responsible or observing the aircraft. If planned, then the controller or observer expects a deviation from a submitted flight plan. Otherwise the pilot has changed course without notifying the appropriate personnel for various reasons.

operational error - An operational error is normally defined as an error which results from the failure to comply with approved procedures which govern the execution and performance of an operation. This failure may be intentional or unintentional.

vehicle or pedestrian deviation - similar definition to pilot deviation except submitted plans or notification of intent is not known or rarely defined. In most cases, declaring a vehicle or pedestrian movement a deviation is de facto, and is noticeable only after a detailed recollection and account of the events.