The flight is considered "departed" from a time-tracking perspective when the boarding doors are closed. If your flight is scheduled at 2pm and they close the doors at 2pm, even if it takes you until 8pm to get in the air, your flight "left on time".

This is a perfect example of "be careful what you measure". Customers don't care when they leave, they care when they arrive, but, since the FAA measures[1] on-time departures, that is what has been optimized for, to the detriment of customers.

1. I would be willing to bet the FAA measures this at the request of the airlines. Measuring arrival time has many variables they can't control, whereas departure, as defined, is mostly controlled by the airlines.

It's because that's one of the metrics which is tracked and, therefore, optimized for, even at the detriment to the core business.

Departure slots. When one comes up you have to be ready to go immediately.

This is why.

Having spent time in the military, it makes sense to me. Hurry up and wait :) I actually like that way of doing things.

Boarded a flight recently and fell asleep at once after sitting down. Woke up three hours later, plane was descending. It was supposed to be a 1.5h flight. Oh well.

In the EU the airlines are legally obligated to provide drink and food if the flight is delayed over a certain amount of time. I believe once you're on the plane/tarmac that clock stops ticking.