This term would be used for someone whom you share an apartment with. It is no longer considerate to call someone a roommate unless they are in fact using your room. But then they would be your girlfriend or boyfriend who unexpectedly just moved in. However, if you live with someone in an apartment they would be your apartmate.
Sally, I'm going out to eat with Mike, my apartmate, tonight and you should join! It'll be fun!
by colormebaeutiful December 13, 2010