A sequel, prequel, or any other addition to a series which just doesn't belong. Created solely to squeeze every last cent out of a successful franchise, and often made by someone other than the original creator.
Sam: Remember the Terminator movies, from when we were kids? Those were the days.
Greg: Remember them? They're still making 'em!
Sam: Nope, I'm pretty sure they're not. All that's left now are tack-ons.