Typically I use the term to refer to that lovely brownish stuff that actual menstrual bleeding tapers off to. I've always assumed it's just whatever tissue/blood is left in the uterus combining with normal discharge. I also say "spotting" in reference to any unscheduled (and unwelcome) traces of that sort of stuff during otherwise normal discharge.
Am I misusing the term, or is it misleading, just an easier and more pleasant way to say "that brownish junk after my period"?
EDIT: Part of this curiosity is me being a linguist. Part of it is that my uterus is responding in unusual-for-me ways to my new Mirena. What I'm currently experiencing seems to be a very light version of the brownish gunk, or maybe just a very slight amount of actual blood, which I figure would actually appear spotty if it weren't combined with regular discharge. (And for the love of god is there an actual medical term I can use instead of saying "brownish gunk"?!)