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