What is defined as a "runway incursion"?

A runway incursion is defined as an incident involving the incorrect presence of a vehicle, person, or aircraft on a runway that can potentially create a hazard for an aircraft taking off or landing. This definition aligns with the answer choice that specifies any unauthorized vehicle or person on an active runway. This concept is vital for maintaining safety at an airport, as runway incursions can lead to serious accidents if not properly managed.

The other options do touch on scenarios that can be related to runway safety but do not fully capture the essence of a runway incursion. For example, while approaching an active runway without clearance is a safety issue, it does not necessarily classify as an incursion unless there is an unauthorized presence on the runway itself. Similarly, crossing a runway without proper radio communication is indicative of unsafe practices, but again, it doesn't directly reference unauthorized presence. Training flights using a runway are part of standard operations and do not qualify as incursions unless they involve unauthorized activities. Therefore, the chosen definition accurately encompasses the core concern of runway incursions within aviation safety.