apartmentmate

Noun

 * 1)  Someone who shares an apartment.
 * 2) * 1991 Spring, Catharine Savage Brosnan, "On husbandry", Sewanee Review, vol. 99 issue 2, page 248:
 * My apartmentmate at the old Rice Institute was the first woman ever to be graduated there with a mechanical engineering degree
 * My apartmentmate at the old Rice Institute was the first woman ever to be graduated there with a mechanical engineering degree

Usage notes
In the UK, the usual term is ; in the US, it is (whether they share a bedroom or not).