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