Seatmate (noun)

A person sitting next to you in a shared seating area, such as on a plane, train, or bus.

Origin:

From seat and mate.

Examples:

1. He was not happy with his seatmate who was taking up too much space.
2. The seatmate was very friendly and they ended up talking the entire flight.
3. The seatmate was a noisy passenger and it made it difficult to sleep.
4. The seatmate was a great companion on the long bus ride.
5. He was assigned a seatmate for the train journey.