die Abfahrt
departure
The time or act of leaving a place by a vehicle such as a train, bus, or ship.
Die Abfahrt des Zuges nach Berlin ist pünktlich um 14:32 Uhr.
The departure of the train to Berlin is right on time at 2:32 p.m.

departure / exit
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departure
The time or act of leaving a place by a vehicle such as a train, bus, or ship.
Die Abfahrt des Zuges nach Berlin ist pünktlich um 14:32 Uhr.
The departure of the train to Berlin is right on time at 2:32 p.m.

'Die Abfahrt' (departure/exit) is the noun form of the separable verb 'abfahren' (to depart/drive off). It describes the act of leaving or a place where one exits from a main route.
Wir müssen jetzt los, damit wir pünktlich abfahren.
We need to leave now so that we depart on time.
In connection with trains, buses, or ships, 'Abfahrt' refers to the exact time when the journey begins. On timetables, it often appears next to 'Ankunft' (arrival).
Bitte beachten Sie die geänderte Zeit für die Abfahrt des Busses.
Please note the changed time for the bus departure.
On the motorway, 'die Abfahrt' is the exit, meaning the road that leads away from the motorway. A typical phrase is 'die nächste Abfahrt nehmen' (to take the next exit).
Wir haben unsere Abfahrt verpasst und müssen jetzt umdrehen.
We missed our exit and now have to turn around.
In sports, especially in winter, 'die Abfahrt' is a discipline in skiing. It refers to skiing quickly down a slope.
Der Skifahrer stürzte bei der Abfahrt und verletzte sich am Knie.
The skier crashed during the downhill race and injured his knee.
start
'der Start' and 'die Abfahrt' can both have to do with the beginning of a trip or journey. 'der Start' is more general and can mean the beginning of a journey, a race, or an activity. 'die Abfahrt' is more specific when a vehicle leaves a place, for example a train, bus, or ship. Learners may confuse the words because both occur in travel contexts. If you are speaking generally about the beginning, 'Start' fits. If it is about the time or process when a vehicle sets off, 'Abfahrt' is more natural.

Die Abfahrt des Zuges ist um 8 Uhr 20 auf Gleis drei.
The train's departure is at 8:20 on platform three.
Der Start der Radtour ist vor der Schule.
The start of the bike tour is in front of the school.