a person in an adjoining seat in an airplane, bus, etc.
seatmate in American English
(ˈsitˌmeit)
noun
a person who shares a seat or occupies the seat next to oneself on a bus, plane, etc
Word origin
[1855–60, Amer.; seat + mate1]This word is first recorded in the period 1855–60. Other words that entered Englishat around the same time include: barrage, keyword, kickoff, pickup, specialist
Examples of 'seatmate' in a sentence
seatmate
She befriends a seatmate who agrees to take the doll into the city (for reasons too boring to describe).
Globe and Mail (2003)
Sitting in the aisle seat means you risk bumping your seatmate whenever you flip the page.