A return route or the return path of a signal/communication; often used in technical or formal contexts (e.g., transport, communications, circuits).
列車の往路と復路では所要時間が異なることがある。
Travel time can differ between the outbound and return train routes.
通信系では復路の遅延が問題になる場合が多い。
In communications systems, delays in the return path are often a problem.
復路はバスで戻る予定だ。
We plan to return by bus.