Flight time is how long a flight takes, measured from the time the aircraft begins to move under it's own power for the purpose of flight, until it comes to rest after the flight. During my training to become a private pilot, we were told to just record the Hobbs time (engine-on time) and use that as flight time. This flight time is usually what is recorded in both the pilot's personal logbook and the aircraft's journey log.
This is not the same as air time, which is the time elapsed from wheels up to wheels down.