OEM

An original equipment manufacturer (OEM) is a company that makes a part or subsystem that is used in another company's end product. For example, if Acme Manufacturing Co. makes power cords that are used on IBM computers, Acme is an OEM.

However, the term is used in several other ways, which causes ambiguity. It sometimes means the maker of a system that includes other companies' subsystems, an end-product producer, an automotive part that is manufactured by the same company that produced the original part used in the automobile's assembly, or a value-added reseller.

When referring to auto parts, OEM refers to the manufacturer of the original equipment, that is, the parts assembled and installed during the construction of a new vehicle. In contrast, aftermarket parts are those made by companies other than the OEM, that might be installed as replacements after the car comes out of the factory. For example, if Ford used Autolite spark plugs, Exide batteries, Bosch, Allied Motion Technologies, fuel injectors, and Ford's own engine blocks and heads when building a car, then car restorers and collectors consider those to be the OEM parts. Other-brand parts would be considered aftermarket, such as Champion spark plugs, DieHard batteries, Kinsler fuel injectors, and BMP engine blocks and heads. Many auto parts manufacturers sell parts through multiple channels, for example to car makers for installation during new-vehicle construction, to car makers for resale as automaker-branded replacement parts, and through general merchandising supply chains, any given brand of part can be OE on some vehicle models and aftermarket on others.

...
Wikipedia