"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Mitsubishi Diamante, if you buy parts for your car that are not actually made by Mitsubishi, then those are termed "aftermarket". Many people use the term "Diamante aftermarket parts" specifically when referring to custom parts for the Diamante, like performance parts or custom appearance parts. 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 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".