The term 'round-trip' can be defined as a journey to a destination and back again. In this crossword, two solutions are provided for the term: 'EXCURSION' (9 letters) and 'ORBIT' (5 letters). An EXCURSION is a short journey or trip, usually for pleasure, while an ORBIT is the path of an object around another object in space. Therefore, both solutions can be used to describe a round-trip journey, either on Earth or in space.