I try to only use that term (at least, the term I'm guessing was behind the stars) in a gender-neutral way, to describe someone who has an unusually large amount of sex with a variety of partners. Eg someone, male or female, who has sex with 10 different people in a week is probably a slvt. Regardless of whether any of the parties involved was married.
Someone who has sex outside of a marriage they are in, without the permission of their spouse, is a cheater, but not necessarily a slvt.
Someone who has sex outside of a marriage they are in *with* the permission of their spouse is... poly or monogamish or something.
Someone who has sex while not married at all is... fairly normal, these days.
And I would treat a serious, long-term relationship equivalently to a marriage, in terms of labeling someone a "cheater" or whatever.