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