A customer refers to any party who is expected to be the purchaser of any goods, deliverables, or services. The term customer can also be used interchangeably with the term user. The customer in terms of project management can be the party (or the purchaser) who is buying the project, meaning, the person who is providing payment for the entirety of services, with promises of a certain set of deliverables to be provided meeting a certain set of criteria. The project management team may also be customers in some scenarios. For example, in the event that the project team chooses to outsource or subcontract any particular element of the project, or a series of activities, then the project management team becomes the customers of that subcontracted party. Services are going to e provided back to the project management team in exchange for a fee of some variety.
This term is defined in the 3rd edition of the PMBOK but not in the 4th.