Aftermarket auto accessories are any accessories manufactured for a car by a company other than the vehicle's manufacturer that are not sourced by the manufacturer, according to automotive website Edmunds. Accessories that are sourced by the original manufacturer are not aftermarket accessories even if they are not installed at purchase.
Aftermarket parts are defined as distinct from parts produced by the vehicle's manufacturer, known as Original Equipment Manufacturer or OEM parts. However, a part manufactured by a company other than the vehicle's manufacturer is still considered an Original Equpiment Manufacturer part if it is sourced by the original manufacturer. For example, factory stereo systems are usually produced by an electronics company rather than the vehicle manufacturer itself, but are not considered aftermarket equipment as a replacement stereo typically is. However, a stereo produced by the same company is still an aftermarket accessory if it is not the same model as the factory stereo itself.
Aftermarket accessories encompass a wide variety of products, from stereo and speaker systems to full body kits that radically change the look of the car. Many accessories are custom designed for specific models of vehicle, so it is important to determine if the accessory in question is compatible with a particular car. Installation of some types of aftermarket accessories also has the potential to void a car's warranty, particularly if the accessory affects the car's mechanical systems.