der Abflug
departure
The takeoff of an airplane from an airport.
Der Abflug nach Mallorca hat sich um eine Stunde verspätet.
The departure to Mallorca has been delayed by an hour.

departure
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departure
The takeoff of an airplane from an airport.
Der Abflug nach Mallorca hat sich um eine Stunde verspätet.
The departure to Mallorca has been delayed by an hour.

'Der Abflug' (departure) is the noun form of the separable verb 'abfliegen' (to depart by plane). The prefix 'ab-' expresses separation or movement away here, which fits the takeoff of an airplane.
Wann fliegen wir morgen ab?
When do we depart tomorrow?
The expression 'den Abflug machen' means colloquially 'to leave' or 'to disappear.' It is mostly used in informal situations among friends or acquaintances.
Die Party ist langweilig, lass uns den Abflug machen.
The party is boring; let's take off.
In the context of airports, you often hear: 'der Abflug verzögert sich' (the departure is delayed), 'die Zeit für den Abflug' (the departure time), or 'der Abflug erfolgt von Gate B24' (the departure is from gate B24).
Bitte beachten Sie die Anzeigetafel für die genaue Zeit Ihres Abflugs.
Please check the display board for the exact time of your departure.
The literal meaning is always connected with travel, airplanes, and airports. You can recognize the colloquial meaning from the fixed structure 'den Abflug machen' (to take off/leave).
Unser Abflug war für 10:00 Uhr geplant.
Our departure was planned for 10:00.
start
'der Start' and 'der Abflug' can both mean the beginning of a flight. 'der Start' more specifically describes the airplane lifting off from the ground. 'der Abflug' more often refers to the scheduled beginning of an air journey at the airport, for example on a ticket or on the departure board. Learners may confuse the words because a takeoff also happens during a departure. For the technical moment in aviation, 'Start' fits. For travel plans and times at the airport, 'Abflug' is more natural.

Der Abflug nach Madrid ist heute um 18 Uhr.
The departure to Madrid is at 6 p.m. today.
Beim Start des Flugzeugs müssen alle Passagiere angeschnallt sein.
During the airplane's takeoff, all passengers must be buckled up.