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