"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Mitsubishi Diamante, if you buy accessories for your car that are not actually made by Mitsubishi, then those are termed "aftermarket". Many people use the term "Diamante aftermarket accessories" specifically when referring to custom accessories for the Diamante, like performance accessories or custom appearance accessories. But even if you just replace your Diamante brake pads with OEM-grade factory-style brake pads that are not made by Mitsubishi, those too are aftermarket Diamante accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".