"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Lamborghini Murcielago, if you buy accessories for your car that are not actually made by Lamborghini, then those are termed "aftermarket". Many people use the term "Murcielago aftermarket accessories" specifically when referring to custom accessories for the Murcielago, like performance accessories or custom appearance accessories. But even if you just replace your Murcielago brake pads with OEM-grade factory-style brake pads that are not made by Lamborghini, those too are aftermarket Murcielago 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".