"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Dodge Omni, if you buy accessories for your car that are not actually made by Dodge, then those are termed "aftermarket". Many people use the term "Omni aftermarket accessories" specifically when referring to custom accessories for the Omni, like performance accessories or custom appearance accessories. But even if you just replace your Omni brake pads with OEM-grade factory-style brake pads that are not made by Dodge, those too are aftermarket Omni accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".