It would seem that there is two definitions of 'market maker' - you've got the literal meaning described by goose etc., but other people use it to refer to prop traders who are permanently in a position, usually taking the other side of the current trend and varying the depth of their 'book' accordingly. I know a couple such guys who refer to themselves as market makers, even though technically they're not; I guess most of the time they are performing the same function.