In a round trip, there are two flights. The second one is the return flight. What's the first one called?
Is it the departure flight? That seems strange, because every flight has a departure and an arrival, doesn't it?
I would call it the "outbound" flight.
By the way I'm not sure "return flight" is unambiguous. In some contexts (such as when booking tickets) it could easily be understood as the combination of an outbound and inbound flight.
(Of course, "outbound" and "inbound" are also slightly ambiguous -- they could refer to legs going from or to an airline hub, but that's more from the perspective of operations planning and timetabling than from the traveler's perspective).
The airline industry uses the term 'Originating Flight' for the first flight in an itinerary.
This could be British but the first thing that came to my mind was "outgoing flight".
Departing flight and returning flight are terms that make sense to me. I note that this is similar to user68196's answer of "originating flight" - the idea is that a return flight is able to refer to a combination of two flights to take you away, and then return you to your original location - so different terms need to be used.