"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Infiniti Q50, if you buy parts for your car that are not actually made by Infiniti, then those are termed "aftermarket". Many people use the term "Q50 aftermarket parts" specifically when referring to custom parts for the Q50, like performance parts or custom appearance parts. But even if you just replace your Q50 brake pads with OEM-grade factory-style brake pads that are not made by Infiniti, those too are aftermarket Q50 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".