A common and technical synonym for "layover" is stopover, though the two terms are often used distinctly in the travel industry. A layover generally refers to a short break between flights, typically lasting less than 24 hours, whereas a stopover often implies a longer stay of 24 hours or more in a middle destination. Other synonyms include connection, hiatus, sojourn, or waypoint. In a more casual or nautical context, one might use the term port of call. If you are looking for a term that emphasizes the waiting aspect, intermission or pause could work, though they are less common in logistics. In the world of aviation and rail, transfer is also frequently used to describe the act of changing vehicles at an intermediate point. Each synonym carries a slightly different nuance regarding the duration and intent of the break in the journey, but "connection" is the most functional alternative for modern air travelers.