What does "taxi-out" signify in aviation terminology?

"Taxi-out" refers specifically to the phase in aviation where the aircraft is in motion on the ground, typically transitioning from the gate or ramp to the runway before takeoff. When the pilots complete the push-back procedure—where the aircraft is moved from its parking position—and begin to navigate the aircraft along the taxiways towards the runway, that is categorized as taxi-out.

This terminology is crucial for understanding the sequence of aircraft movements and the operational phases during a flight. It indicates that the aircraft is preparing for takeoff, thereby signaling to air traffic control and other personnel that the flight is progressing towards departure.

The other choices focus on different stages of flight operation or passenger service. Climbing to altitude represents a post-takeoff condition, arriving at the terminal indicates landing and taxiing back to the gate, and passenger boarding procedures occur prior to push-back and taxi-out. Each of these stages precedes or follows the "taxi-out" phase, showcasing its specific role in the flight process.