Seatmate is a common term used when referring to the person sitting beside you. However, there are various synonyms for this term that can be used to describe your fellow passenger in different settings. For example, if you are in a classroom or at an event, you might refer to them as your companion, neighbor, or partner. In a more formal setting such as a conference or a meeting, you might use the term colleague or fellow attendee. If you are on a plane or train, you might refer to them as your co-passenger or travel buddy. Overall, the term seatmate may be the most commonly used term, but there are plenty of other words to describe the person sitting next to you.