"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Dodge Omni, if you buy parts for your car that are not actually made by Dodge, then those are termed "aftermarket". Many people use the term "Omni aftermarket parts" specifically when referring to custom parts for the Omni, like performance parts or custom appearance parts. 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 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".