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