"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Lotus Elise, if you buy parts for your car that are not actually made by Lotus, then those are termed "aftermarket". Many people use the term "Elise aftermarket parts" specifically when referring to custom parts for the Elise, like performance parts or custom appearance parts. 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 parts. Over the course of a vehicle's life, it almost certainly will have aftermarket parts installed in it at some point, even if the vehicle is never actually "modded out".