From Longman Dictionary of Contemporary Englishsteady boyfriend/girlfriendsteady boyfriend/girlfriendGIRLFRIEND/BOYFRIENDsomeone that you have been having a romantic relationship with for a long time → steady
Examples from the Corpus
steady boyfriend/girlfriend• In any case, life was much too enjoyable to think about steady boyfriends.• It would have to be with a steady boyfriend.• Then, he started getting interested in banger-racing and got a steady girlfriend.• Now at 16 Steven has a steady girlfriend of 15, who he has been going out with for several months.• The first year at university I had a very steady boyfriend who was doing a similar course as me but mechanical engineering. 