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