"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Isuzu Axiom, if you buy accessories for your car that are not actually made by Isuzu, then those are termed "aftermarket". Many people use the term "Axiom aftermarket accessories" specifically when referring to custom accessories for the Axiom, like performance accessories or custom appearance accessories. But even if you just replace your Axiom brake pads with OEM-grade factory-style brake pads that are not made by Isuzu, those too are aftermarket Axiom accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".