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