What is the definition of flight time in aviation?

Flight time in aviation is defined as the duration from when the aircraft's engines start until they are shut down. This encompasses all the time that the aircraft is in operation, including taxiing, takeoff, flight, and landing. Therefore, the correct answer accurately reflects this definition, as it captures the comprehensive period during which the aircraft is actively being operated by the crew.

The other options do not encapsulate the complete scope of flight time. For instance, taking the period from takeoff until landing only accounts for the airborne portion and ignores the time spent on the ground with engines running before takeoff or after landing. Defining flight time strictly as the ascent to cruising altitude misses out on essential moments of engine operation. Lastly, focusing solely on the duration of taxiing does not account for the actual flight operations that occur beyond this phase. Thus, the answer aligns precisely with aviation regulations and standard definitions that govern flight operations.