IPA: //ˈflæt.meɪt//
KK: /ˈflætˌmeɪt/
A person who lives in the same apartment or rental property as someone else, sharing the space and expenses.
I have a flatmate who is studying at the university.
Flatmate → It is formed from "flat" (from Old English *flet*, meaning a dwelling or living space) and "mate" (from Old English *gemetta*, meaning companion or associate). The word "flatmate" refers to a companion or associate who shares a living space or dwelling.
Think of a 'flat' as a living space and 'mate' as a companion — a flatmate is someone who shares that living space with you.