Refers to someone you live with but not necessarily share a family or romantic relationship with. My housemate and I split the rent and utilities.
Pro Tip 2/3
Not Always Friends
Housemates might simply be co-inhabitants with little social interaction outside sharing living spaces. Even though we're housemates, we rarely hang out together.
Pro Tip 3/3
Shared Responsibilities
Implies that there's an unwritten or formal agreement to share household duties and expenses. My housemate is responsible for cleaning the kitchen every week.