Original Equipment Manufacturer. Refers to factory parts made by the same manufacturer that supplied the original vehicle. Counterpart to aftermarket.
OEM stands for Original Equipment Manufacturer. The term refers to factory parts produced by the same manufacturer (or supplier under contract) that supplied the original vehicle. OEM parts are the same parts the vehicle came with from the factory. The counterpart is aftermarket, which refers to parts produced by independent manufacturers.
OEM parts have specific advantages: they are designed to factory specifications, they fit perfectly, they are warrantied by the manufacturer, and they are supported by dealer service networks. The disadvantages are higher cost (often 50-200% more than aftermarket equivalents) and limited choice (only one design typically available). Aftermarket parts may offer different price points, different design philosophies, or performance enhancements not available in OEM.
OEM-plus is a related term meaning "OEM parts from higher trims or related models." For example, fitting Civic Type R parts to a base Civic LX is OEM-plus modification: all parts are factory Honda parts, but they are upgrades from the original specification. This is often preferred over pure aftermarket because the parts are factory-engineered and tested.