What is a roundtrip flight?
A roundtrip flight is an itinerary in which you return to the initial departure city (e.g. flying from New York to Los Angeles, then returning back to New York).
Long-haul roundtrip flights with major carriers are almost always cheaper than booking two one-ways.
Can I fly only one segment of a roundtrip itinerary?
You can, but only if you don’t miss any flights prior.
Standard aviation rules state that if you miss any segment within a booking, the rest of your ticket is automatically cancelled.
This means you would not be able to only fly the last segment of a trip, because by that point you are a no-show for the earlier flights and so your ticket is cancelled.
Can you provide an example
Sure. Let’s say you’ve booked a cheap roundtrip flight from Dallas to Dubai with Lufthansa which involves a stop in Frankfurt.
You could fly the one-way Dallas – Frankfurt – Dubai and simply not show up for the return flights. There’s nothing stopping you from doing that.
However, if you only wanted to use the Dubai – Frankfurt – Dallas flights, that would not be possible. Because when you failed to show up for the first flight (Dallas to Frankfurt), the airline would put you down as a no-show and the rest of your booking would be cancelled.