What does it mean for an aircraft to be considered in operation?

An aircraft is considered "in operation" when it first moves under its own power for takeoff, which signifies the transition from being an inactive object to actively participating in a flight. This status implies that the aircraft is no longer stationary and is actively engaged in the flight process. It reflects the operational phase of flight where the aircraft's systems, including navigation and communication, are functioning, and crew members are performing their duties to ensure a safe takeoff.

When looking at the other contexts, being stationary on the ground reflects a state of inactivity, and while boarding passengers is a crucial step in the flight process, it does not denote the aircraft being in operation from an aviation perspective. Similarly, while the engines running is an essential part of preparing for flight, it does not equate to the aircraft being operational unless it is actively moving towards takeoff.