IPA: //ˈsiːtˌmeɪt//
KK: /ˈsiːtˌmeɪt/
A person who sits next to someone else, especially on a bus, train, or airplane.
My seatmate on the flight was very friendly and we talked the whole way.
Seatmate → It is formed from "seat" (from Old English "sæte", meaning a place to sit) and "mate" (from Old English "gemetta", meaning companion or friend). The word "seatmate" refers to a companion or friend who shares a seat with someone.
Think of someone who shares a 'place to sit' ('seat') with you — that's your 'mate' or companion.