What used to be called mistress now days is called a girlfriend so we are left with a puzzle, is there such a thing as a mistress these days? Does it only apply to married people? Yes, definitely a puzzle.
Yes there are mistresses today. A mistress is a women whose only income is supplied by a man and whose job it is to please a man. These women do not have wealth of their own but are "kept" by the man. In essence they are paid to play. There is a difference between a mistress and a women of another sort in that there is a relationship between the man and his mistress etc. You get the picture. A girlfriend usually has her own life and unless independantly wealthy has a job and pays for her own apartment, clothes etc. or if she is in a "live in" situation usually details of who covers rent etc are worked out. A women with no job and no wealth who lives with her male companion is usually referred to as a mistress or in polite terms may also be called companion or live-in companion , but again companions usally have a job or are financially independant in most cases. It's a slippery slope once you are a woman of 30 or older with no job, no money of one's own and not married.