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