What does it mean when an aircraft is not in operation?

When an aircraft is described as not being in operation, it indicates that the aircraft is stationary on the ground, meaning it is not actively flying, taking off, or taxiing. This status can occur when the aircraft is parked at the gate, in a hangar, or otherwise not engaged in any flight-related activities.

Other options, such as taxiing on the runway, indicate that the aircraft is still in motion and actively involved in a phase of operation, whether it be preparing for takeoff or returning to a parking position. Being about to take off signifies that the aircraft is preparing for the flight phase, while being loaded with passengers suggests that the aircraft is engaged in operational activities, preparing for a scheduled flight. Therefore, the correct understanding of "not in operation" distinctly applies to the aircraft being stationary, as it is disconnected from any flight processes.