A person with whom one shares a rental property.
AKA a room mate or roomie.
Someone to split the rent with. (If they have the money)
Tom: Whoa, Somebody broke in and trashed your apartment while you were gone!
You: I wish. No, it was just my Flat mate, he's a pig.
You: Hey will you wash the dishes?
Flat mate: Ignores you and goes to his room.