The concept of the project management office refers to the particular organizational body and or entity that ultimately is the hub and or center of the planning and activity of a particular project. The project management office is ultimately assigned a number of distinct and varied responsibilities as related to the centralized and ultimately coordinated management of the entirety of the number of projects that may fall within and under its domain. The ultimate list of responsibilities that may fall underneath the umbrella header of the project management office (also known by the three letter anagram PMO) can range from something as mundane as providing project management support functions to the actual element of being responsible for some particular elements of the direct management of the project. For a further understanding of this, if may also be helpful to review the definition of program management office.
This term is defined in the 3rd and the 4th edition of the PMBOK.