"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Datsun 260Z, if you buy parts for your car that are not actually made by Datsun, then those are termed "aftermarket". Many people use the term "260Z aftermarket parts" specifically when referring to custom parts for the 260Z, like performance parts or custom appearance parts. But even if you just replace your 260Z brake pads with OEM-grade factory-style brake pads that are not made by Datsun, those too are aftermarket 260Z 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".