The close project process is essentially the process in which all activities, work, and tasks that make up the entirety of a project are finalized. More specifically, it typically refers to the closing of all activities across all project groups. The close project process can take place either before the deliverables have been submitted, which is often times a perquisite of deliverable acceptance, however, if it is possible, it is ideal to initiate the close project process after the deliverables have been accepted and approved by the assigning party. The reason for this is because, in the event the deliverables are not approved, the assigning party may request that the project team reexamine some elements of the deliverables and potentially make modifications. If the close project process has been initiated, and even more so if it has been completed, it may be far more difficult to restart the project activity necessary to make these requested adjustments.
This term is defined in the 3rd edition of the PMBOK but not in the 4th.