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