This term, commonly used by college-age students, is someone you share a house with (or, less commonly, an apartment).
These people were, to my knowledge, formerly known as "roommates," in virtually all contexts.
Housemate seems to be merely one of the many trendy terms being tossed around these days.
Here are possible reasons for this word's existence:
A) Perhaps it's supposed to have less of the negative stigma that the word "roommate" seems to carry with it.
B) Also possible is that roommate implies a closer relationship/friendship than simply housemate.
John, Dave, and Bill were looking for another housemate to share in the rent.
That guy across the hall, he's my housemate.
to have sex with a person who participates in community living.
My friend just housemated with my fellow housemate.
Watch as the majestic female lioness housemates with the pale, troll of a cooper (communal living practitioner), also known as my housemate Jim.