OEM is an acronym for Original Equipment Manufacturer.
An OEM is a company that builds systems (often related to electronics, e.g. computers) using components bought in bulk from other manufacturers.
Note that the OEM is the company building the system, not the component manufacturer. This is a common confusion.
In common language the acronym OEM is often used when value-added reseller is the actual word sought for. This is the case when a piece of equipment is designed and manufactured by one company, but sold under the brand of another company.