I have a question about what "departure time" actually is, when you look at information on Websites like Flight Aware. Is it the time the plane leaves the gate, or the time it actually gets airborne?

For example, let's say a flight was originally scheduled to leave at 7pm, and the departure time on Flight Aware states 8pm (after the fact). Does it mean the flight could have closed at, say, 6:45PM, and the plane just taxied out and sat there on the tarmac for an hour? Or does it mean the flight didn't close until just before 8PM? Thanks.