Aftermarket is an umbrella term for the collective network of vendors who design and sell vehicular components that are intended to replace the stock manufacturer's parts in order to alter the appearance or performance of the vehicle. Alternatively, an aftermarket part can be a straight replacement for a stock item at a lower price with no intention to cause such a change.

The criteria used to design a vehicle are based in large part on the features that would sell to the widest audience at a reasonable price for the vehicle's class. Reliability, price, and fuel economy are typically factors in the decision-making process.

The aftermarket has become the means by which one may customize one's vehicle to suit a particular personality, to provide a role not satisfied by the stock vehicle, or to simply stand out. Aggressive styling and/or performance enhancements, which might not be important to the average buyer, usually make up a significant portion of the aftermarket. In fact, some parts come bundled together within a larger kit that one may choose to install in order to alter one's vehicle to a more significant degree.